Tonatiuh Rangel - Academia.edu (original) (raw)
Papers by Tonatiuh Rangel
The Journal of Chemical Physics
ABINIT is probably the first electronic-structure package to have been released under an open-sou... more ABINIT is probably the first electronic-structure package to have been released under an open-source license, about twenty years ago. It implements density functional theory (DFT), density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpether equation), and more specific or advanced formalisms, like dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" (TDEP) approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAW), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which ABINIT relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages, either among planewave codes, or in general: treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; DFPT calculations of response to strain perturbation (elastic constants, piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, spin-magnetic-field perturbation. The ABINIT DFPT implementation is very general, including systems with van der Waals interaction, or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW data sets, high-throughput calculations (databases of phonon
2008 Digest of the Leos Summer Topical Meetings, 2008
We present a study of optical electron spin-injection at the surface of semiconductors from direc... more We present a study of optical electron spin-injection at the surface of semiconductors from direct optical excitation with circularly polarized light.
The Journal of chemical physics, Jan 21, 2017
Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with... more Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with (i) the consistent-exchange vdW-DF-cx functional [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] and (ii) with the vdW-DF2 functional [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. The ability to describe covalent and non-covalent binding properties of molecules is assessed. For properties related to covalent binding, atomization energies (G2-1 set), molecular reaction energies (G2RC set), and ionization energies (G21IP set) are benchmarked against experimental reference values. We find that hybrid-vdW-DF-cx yields results that are rather similar to those of the standard non-empirical hybrid PBE0 [C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999)], with mean average deviations (MADs) of 4.9 and 5.0 kcal/mol for the G2-1 set, respectively. In this comparison, experimental reference values are used, back corrected by wavefunction-based quantum-chemistry calculations ...
Physical Review B
Organic molecular crystals are expected to feature appreciable electron-phonon interactions that ... more Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. In this work we report first principles calculations and analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence and conduction band edges employing density functional theory. Further, our calculated phonon renormalization of the GW-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperatures. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.
Computer Physics Communications
Ab initio many-body perturbation theory within the GW approximation is a Green's function formali... more Ab initio many-body perturbation theory within the GW approximation is a Green's function formalism widely used in the calculation of quasiparticle excitation energies of solids. In what has become an increasingly standard approach, Kohn-Sham eigenenergies, generated from a DFT calculation with a strategically-chosen exchange correlation functional "starting point", are used to construct G and W , and then perturbatively corrected by the resultant GW self-energy. In practice, there are several ways to construct the GW self-energy, and these can lead to variations in predicted quasiparticle energies. For example, for ZnO and TiO2, reported GW fundamental gaps can vary by more than 1 eV. In this work, we address the convergence and key approximations in contemporary G0W0 calculations, including frequency-integration schemes and the treatment of the Coulomb divergence in the exact-exchange term. We study several systems, and compare three different GW codes: BerkeleyGW, Abinit and Yambo. We demonstrate, for the first time, that the same quasiparticle energies for systems in the condensed phase can be obtained with different codes, and we provide a comprehensive assessment of implementations of the GW approximation.
Physical Review B
We study the injection current response tensor (also known as circular photogalvanic effect or ba... more We study the injection current response tensor (also known as circular photogalvanic effect or ballistic current) in ferrolectric monolayer GeS, GeSe, SnS, and SnSe. We find that the injection current is perpendicular to the spontaneous in-plane polarization and could reach peak (bulk) values of the order of 10 10 A/V 2 s in the visible spectrum. The magnitude of the injection current is the largest reported in the literature to date for a two dimensional material. To rationalize the large injection current, we correlate the injection current spectrum with the joint density of states, electric polarization, strain, etc. We find that various factors such as anisotropy, in-plane polarization and wave function delocalization are important in determining the injection current tensor in these materials. We also find that compression along the polar axis can increase the injection current (or change its sign), and hence strain can be an effective control knob for their nonlinear optical response. Conversely, the injection current can be a sensitive probe of the crystal structure.
Physical Review Materials
Halide perovskites constitute a chemically-diverse class of crystals with great promise as photov... more Halide perovskites constitute a chemically-diverse class of crystals with great promise as photovoltaic absorber materials, featuring band gaps between about 1 and 3.5 eV depending on composition. Their diversity calls for a general computational approach to predicting their band gaps. However, such an approach is still lacking. Here, we use density functional theory (DFT) and ab initio many-body perturbation theory within the GW approximation to compute the quasiparticle or fundamental band gap of a set of ten representative halide perovskites: CH3NH3PbI3 (MAPbI3), MAPbBr3, CsSnBr3, (MA)2BiTlBr6, Cs2TlAgBr6, Cs2TlAgCl6, Cs2BiAgBr6, Cs2InAgCl6, Cs2SnBr6, and Cs2Au2I6. Comparing with recent measurements, we find that a standard generalized gradient exchange-correlation functional can significantly underestimate the experimental band gaps of these perovskites, particularly in cases with strong spin-orbit coupling (SOC) and highly dispersive band edges, to a degree that varies with composition. We show that these nonsystematic errors are inherited by one-shot G0W0 and eigenvalue self-consistent GW0 calculations, demonstrating that semilocal DFT starting points are insufficient for MAPbI3, MAPbBr3, CsSnBr3, (MA)2BiTlBr6, Cs2TlAgBr6, and Cs2TlAgCl6. On the other hand, we find that DFT with hybrid functionals leads to an improved starting point and GW0 results in better agreement with experiment for these perovskites. Our results suggest that GW0 with hybrid functional-based starting points are promising for predicting band gaps of systems with large SOC and dispersive bands in this technologically important class of semiconducting crystals.
Chemical Science
Correction for ‘Enhancement of CO2 binding and mechanical properties upon diamine functionalizati... more Correction for ‘Enhancement of CO2 binding and mechanical properties upon diamine functionalization of M2(dobpdc) metal–organic frameworks’ by Jung-Hoon Lee et al., Chem. Sci., 2018, 9, 5197–5206.
Physical Review B
Many-body perturbation theory within the GW approach has been established as a quantitatively acc... more Many-body perturbation theory within the GW approach has been established as a quantitatively accurate approach for predicting the quasiparticle and excited-state properties of a wide variety of materials. However, the successful application of the method is often complicated by the computational complexity associated with the evaluation and inversion of the frequency-dependent dielectric matrix ε(ω). Here, we describe an approach to speed up the evaluation of the frequencydependent part of ε(ω) in the traditional sum-overstates GW framework based on the low-rank approximation of the static dielectric matrix, a technique often used in GW implementations that are based on a starting mean field within density-functional perturbation theory. We show that the overall accuracy of the approach, independently from other calculation parameters, is solely determined by the threshold on the eigenvalues of the static dielectric matrix, ε(ω=0), and that it can yield orders-of-magnitude speed-ups in full-frequency GW calculations. We validate our implementation with several benchmark calculations ranging from bulk materials to systems with reduced dimensionality, and show that this technique allows one not only to study larger systems, but also to carefully consider the convergence of computationally demanding systems, such as ZnO, without relying on plasmon-pole models.
Proceedings of the National Academy of Sciences
Organic materials are promising candidates for advanced optoelectronics and are used in light-emi... more Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with first-principles calculations for the absorption based on the GW plus Bethe–Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying e...
The Journal of Physical Chemistry A
Chemical Science
We predict that the orientationally-averaged Young's modulus of mmen–Zn2(dobpdc) increases by... more We predict that the orientationally-averaged Young's modulus of mmen–Zn2(dobpdc) increases by 112% compared to Zn2(dobpdc), a remarkable increase.
Physical Review B
Acene molecular crystals are of current interest in organic optoelectronics, both as active mater... more Acene molecular crystals are of current interest in organic optoelectronics, both as active materials and for exploring and understanding new phenomena. Phonon scattering can be an important facilitator and dissipation mechanism in charge separation and carrier transport processes. Here, we carry out density functional theory (DFT) calculations of the structure and the full phonon dispersion of crystalline naphthalene, a well-characterized acene crystal for which detailed neutron diffraction measurements, as well as infrared and Raman spectroscopy, are available. We evaluate the performance, relative to experiments, of the local density approximation (LDA); the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE); and a recent van der Waalscorrected non-local correlation functional (vdW-DF-cx). We find that the vdW-DF-cx functional accurately predicts lattice parameters of naphthalene within 1%. Intermolecular and intramolecular phonon frequencies across the Brillouin zone are reproduced within 7.8% and 1%, respectively. As expected, LDA (PBE) underestimates (overestimates) the lattice parameters and overestimates (underestimates) phonon frequencies, demonstrating their shortcomings for predictive calculations of weakly-bound materials. Additionally, if the unit cell is fixed to the experimental lattice parameters, PBE is shown to lead to improved phonon frequencies. Our study provides a detailed understanding of the phonon spectrum of naphthalene, and highlights the importance of including van der Waals dispersion interactions in predictive calculations of lattice parameters and phonon frequencies of molecular crystals and related organic materials.
Physical Review B
Using density functional theory (DFT) with van der Waals functionals, we calculate the adsorption... more Using density functional theory (DFT) with van der Waals functionals, we calculate the adsorption energetics and geometry of benzenediamine (BDA) molecules on Au(111) surfaces. Our results demonstrate that the reported self-assembled linear chain structure of BDA, stabilized via hydrogen bonds between amine groups, is energetically favored over previously-studied monomeric phases. Moreover, using a model, which includes nonlocal polarization effects from the substrate and the neighboring molecules and incorporates many-body perturbation theory calculations within the GW approximation, we obtain approximate self-energy corrections to the DFT highest occupied molecular orbital (HOMO) energy associated with BDA adsorbate phases. We find that, independent of coverage, the HOMO energy of the linear chain phase is lower relative to the Fermi energy than that of the monomer phase, and in good agreement with values measured with ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy.
Computer Physics Communications
We summarize the molgw code that implements density-functional theory and many-body perturbation ... more We summarize the molgw code that implements density-functional theory and many-body perturbation theory in a Gaussian basis set. The code is dedicated to the calculation of the many-body self-energy within the GW approximation and the solution of the Bethe-Salpeter equation. These two types of calculations allow the user to evaluate physical quantities that can be compared to spectroscopic experiments. Quasiparticle energies, obtained through the calculation of the GW self-energy, can be compared to photoemission or transport experiments, and neutral excitation energies and oscillator strengths, obtained via solution of the Bethe-Salpeter equation, are measurable by optical absorption. The implementation choices outlined here have aimed at the accuracy and robustness of calculated quantities with respect to measurments. Furthermore, the algorithms implemented in molgw allow users to consider molecules or clusters containing up to 100 atoms with rather accurate basis sets, and to choose whether or not to apply the resolution-of-the-identity approximation. Finally, we demonstrate the parallelization efficacy of the molgw code over several hundreds of processors.
Physical Review Letters
We use a first-principles density functional theory approach to calculate the shift current and l... more We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current (∼100 µA/V 2), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective threedimensional electric polarization (∼1.9 C/m 2). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials
Journal of chemical theory and computation, Jan 14, 2016
Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technolo... more Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technological applications, are widely studied and therefore provide an excellent system for benchmarking theoretical methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approximation relative to new high-quality CCSD(T) reference data for charged excitations of the acenes. We compare GW calculations with a number of hybrid density functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with molecule length increasing from benzene to hexacene. We find that GW calculations with starting points based on an optimally tuned range-separated hybrid (OTRSH) density functional and eigenvalue self-consistency can yield quantitative ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from reference...
The Journal of chemical physics, Jan 21, 2017
The accurate prediction of singlet and triplet excitation energies is an area of intense research... more The accurate prediction of singlet and triplet excitation energies is an area of intense research of significant fundamental interest and critical for many applications. Most calculations of singlet and triplet energies use time-dependent density functional theory (TDDFT) in conjunction with an approximate exchange-correlation functional. In this work, we examine and critically assess an alternative method for predicting low-lying neutral excitations with similar computational cost, the ab initio Bethe-Salpeter equation (BSE) approach, and compare results against high-accuracy wavefunction-based methods. We consider singlet and triplet excitations of 27 prototypical organic molecules, including members of Thiel's set, the acene series, and several aromatic hydrocarbons exhibiting charge-transfer-like excitations. Analogous to its impact in TDDFT, we find that the Tamm-Dancoff approximation (TDA) overcomes triplet instabilities in the BSE approach, improving both triplet and sing...
Physical Review B, 2016
The Ruddlesden-Popper (RP) homologous series Srn+1TinO3n+1 provides a useful template for the stu... more The Ruddlesden-Popper (RP) homologous series Srn+1TinO3n+1 provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO3. We use ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectrum of Srn+1TinO3n+1 for n = 1 − 5 and ∞. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small n and reach a value of 330 meV for n = 1, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr2TiO4 (n = 1) localizes in the 2D plane defined by the TiO2 layer, and explain the origin of its localization.
Physical Review B, 2016
Molecular crystals are a prototypical class of van der Waals (vdW)-bound organic materials with e... more Molecular crystals are a prototypical class of van der Waals (vdW)-bound organic materials with excited state properties relevant for optoelectronics applications. Predicting the structure and excited state properties of molecular crystals presents a challenge for electronic structure theory, as standard approximations to density functional theory (DFT) do not capture long-range vdW dispersion interactions and do not yield excited-state properties. In this work, we use a combination of DFT including vdW forces-using both non-local correlation functionals and pair-wise correction methods-together with many-body perturbation theory (MBPT) to study the geometry and excited states, respectively, of the entire series of oligoacene crystals, from benzene to hexacene. We find that vdW methods can predict lattice constants within 1% of the experimental measurements, on par with the previously reported accuracy of pair-wise approximations for the same systems. We further find that excitation energies are sensitive to geometry, but if optimized geometries are used MBPT can yield excited state properties within a few tenths of an eV from experiment. We elucidate trends in MBPT-computed charged and neutral excitation energies across the acene series and discuss the role of common approximations used in MBPT.
The Journal of Chemical Physics
ABINIT is probably the first electronic-structure package to have been released under an open-sou... more ABINIT is probably the first electronic-structure package to have been released under an open-source license, about twenty years ago. It implements density functional theory (DFT), density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpether equation), and more specific or advanced formalisms, like dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" (TDEP) approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAW), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which ABINIT relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages, either among planewave codes, or in general: treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; DFPT calculations of response to strain perturbation (elastic constants, piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, spin-magnetic-field perturbation. The ABINIT DFPT implementation is very general, including systems with van der Waals interaction, or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW data sets, high-throughput calculations (databases of phonon
2008 Digest of the Leos Summer Topical Meetings, 2008
We present a study of optical electron spin-injection at the surface of semiconductors from direc... more We present a study of optical electron spin-injection at the surface of semiconductors from direct optical excitation with circularly polarized light.
The Journal of chemical physics, Jan 21, 2017
Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with... more Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with (i) the consistent-exchange vdW-DF-cx functional [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] and (ii) with the vdW-DF2 functional [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. The ability to describe covalent and non-covalent binding properties of molecules is assessed. For properties related to covalent binding, atomization energies (G2-1 set), molecular reaction energies (G2RC set), and ionization energies (G21IP set) are benchmarked against experimental reference values. We find that hybrid-vdW-DF-cx yields results that are rather similar to those of the standard non-empirical hybrid PBE0 [C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999)], with mean average deviations (MADs) of 4.9 and 5.0 kcal/mol for the G2-1 set, respectively. In this comparison, experimental reference values are used, back corrected by wavefunction-based quantum-chemistry calculations ...
Physical Review B
Organic molecular crystals are expected to feature appreciable electron-phonon interactions that ... more Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. In this work we report first principles calculations and analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence and conduction band edges employing density functional theory. Further, our calculated phonon renormalization of the GW-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperatures. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.
Computer Physics Communications
Ab initio many-body perturbation theory within the GW approximation is a Green's function formali... more Ab initio many-body perturbation theory within the GW approximation is a Green's function formalism widely used in the calculation of quasiparticle excitation energies of solids. In what has become an increasingly standard approach, Kohn-Sham eigenenergies, generated from a DFT calculation with a strategically-chosen exchange correlation functional "starting point", are used to construct G and W , and then perturbatively corrected by the resultant GW self-energy. In practice, there are several ways to construct the GW self-energy, and these can lead to variations in predicted quasiparticle energies. For example, for ZnO and TiO2, reported GW fundamental gaps can vary by more than 1 eV. In this work, we address the convergence and key approximations in contemporary G0W0 calculations, including frequency-integration schemes and the treatment of the Coulomb divergence in the exact-exchange term. We study several systems, and compare three different GW codes: BerkeleyGW, Abinit and Yambo. We demonstrate, for the first time, that the same quasiparticle energies for systems in the condensed phase can be obtained with different codes, and we provide a comprehensive assessment of implementations of the GW approximation.
Physical Review B
We study the injection current response tensor (also known as circular photogalvanic effect or ba... more We study the injection current response tensor (also known as circular photogalvanic effect or ballistic current) in ferrolectric monolayer GeS, GeSe, SnS, and SnSe. We find that the injection current is perpendicular to the spontaneous in-plane polarization and could reach peak (bulk) values of the order of 10 10 A/V 2 s in the visible spectrum. The magnitude of the injection current is the largest reported in the literature to date for a two dimensional material. To rationalize the large injection current, we correlate the injection current spectrum with the joint density of states, electric polarization, strain, etc. We find that various factors such as anisotropy, in-plane polarization and wave function delocalization are important in determining the injection current tensor in these materials. We also find that compression along the polar axis can increase the injection current (or change its sign), and hence strain can be an effective control knob for their nonlinear optical response. Conversely, the injection current can be a sensitive probe of the crystal structure.
Physical Review Materials
Halide perovskites constitute a chemically-diverse class of crystals with great promise as photov... more Halide perovskites constitute a chemically-diverse class of crystals with great promise as photovoltaic absorber materials, featuring band gaps between about 1 and 3.5 eV depending on composition. Their diversity calls for a general computational approach to predicting their band gaps. However, such an approach is still lacking. Here, we use density functional theory (DFT) and ab initio many-body perturbation theory within the GW approximation to compute the quasiparticle or fundamental band gap of a set of ten representative halide perovskites: CH3NH3PbI3 (MAPbI3), MAPbBr3, CsSnBr3, (MA)2BiTlBr6, Cs2TlAgBr6, Cs2TlAgCl6, Cs2BiAgBr6, Cs2InAgCl6, Cs2SnBr6, and Cs2Au2I6. Comparing with recent measurements, we find that a standard generalized gradient exchange-correlation functional can significantly underestimate the experimental band gaps of these perovskites, particularly in cases with strong spin-orbit coupling (SOC) and highly dispersive band edges, to a degree that varies with composition. We show that these nonsystematic errors are inherited by one-shot G0W0 and eigenvalue self-consistent GW0 calculations, demonstrating that semilocal DFT starting points are insufficient for MAPbI3, MAPbBr3, CsSnBr3, (MA)2BiTlBr6, Cs2TlAgBr6, and Cs2TlAgCl6. On the other hand, we find that DFT with hybrid functionals leads to an improved starting point and GW0 results in better agreement with experiment for these perovskites. Our results suggest that GW0 with hybrid functional-based starting points are promising for predicting band gaps of systems with large SOC and dispersive bands in this technologically important class of semiconducting crystals.
Chemical Science
Correction for ‘Enhancement of CO2 binding and mechanical properties upon diamine functionalizati... more Correction for ‘Enhancement of CO2 binding and mechanical properties upon diamine functionalization of M2(dobpdc) metal–organic frameworks’ by Jung-Hoon Lee et al., Chem. Sci., 2018, 9, 5197–5206.
Physical Review B
Many-body perturbation theory within the GW approach has been established as a quantitatively acc... more Many-body perturbation theory within the GW approach has been established as a quantitatively accurate approach for predicting the quasiparticle and excited-state properties of a wide variety of materials. However, the successful application of the method is often complicated by the computational complexity associated with the evaluation and inversion of the frequency-dependent dielectric matrix ε(ω). Here, we describe an approach to speed up the evaluation of the frequencydependent part of ε(ω) in the traditional sum-overstates GW framework based on the low-rank approximation of the static dielectric matrix, a technique often used in GW implementations that are based on a starting mean field within density-functional perturbation theory. We show that the overall accuracy of the approach, independently from other calculation parameters, is solely determined by the threshold on the eigenvalues of the static dielectric matrix, ε(ω=0), and that it can yield orders-of-magnitude speed-ups in full-frequency GW calculations. We validate our implementation with several benchmark calculations ranging from bulk materials to systems with reduced dimensionality, and show that this technique allows one not only to study larger systems, but also to carefully consider the convergence of computationally demanding systems, such as ZnO, without relying on plasmon-pole models.
Proceedings of the National Academy of Sciences
Organic materials are promising candidates for advanced optoelectronics and are used in light-emi... more Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with first-principles calculations for the absorption based on the GW plus Bethe–Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying e...
The Journal of Physical Chemistry A
Chemical Science
We predict that the orientationally-averaged Young's modulus of mmen–Zn2(dobpdc) increases by... more We predict that the orientationally-averaged Young's modulus of mmen–Zn2(dobpdc) increases by 112% compared to Zn2(dobpdc), a remarkable increase.
Physical Review B
Acene molecular crystals are of current interest in organic optoelectronics, both as active mater... more Acene molecular crystals are of current interest in organic optoelectronics, both as active materials and for exploring and understanding new phenomena. Phonon scattering can be an important facilitator and dissipation mechanism in charge separation and carrier transport processes. Here, we carry out density functional theory (DFT) calculations of the structure and the full phonon dispersion of crystalline naphthalene, a well-characterized acene crystal for which detailed neutron diffraction measurements, as well as infrared and Raman spectroscopy, are available. We evaluate the performance, relative to experiments, of the local density approximation (LDA); the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE); and a recent van der Waalscorrected non-local correlation functional (vdW-DF-cx). We find that the vdW-DF-cx functional accurately predicts lattice parameters of naphthalene within 1%. Intermolecular and intramolecular phonon frequencies across the Brillouin zone are reproduced within 7.8% and 1%, respectively. As expected, LDA (PBE) underestimates (overestimates) the lattice parameters and overestimates (underestimates) phonon frequencies, demonstrating their shortcomings for predictive calculations of weakly-bound materials. Additionally, if the unit cell is fixed to the experimental lattice parameters, PBE is shown to lead to improved phonon frequencies. Our study provides a detailed understanding of the phonon spectrum of naphthalene, and highlights the importance of including van der Waals dispersion interactions in predictive calculations of lattice parameters and phonon frequencies of molecular crystals and related organic materials.
Physical Review B
Using density functional theory (DFT) with van der Waals functionals, we calculate the adsorption... more Using density functional theory (DFT) with van der Waals functionals, we calculate the adsorption energetics and geometry of benzenediamine (BDA) molecules on Au(111) surfaces. Our results demonstrate that the reported self-assembled linear chain structure of BDA, stabilized via hydrogen bonds between amine groups, is energetically favored over previously-studied monomeric phases. Moreover, using a model, which includes nonlocal polarization effects from the substrate and the neighboring molecules and incorporates many-body perturbation theory calculations within the GW approximation, we obtain approximate self-energy corrections to the DFT highest occupied molecular orbital (HOMO) energy associated with BDA adsorbate phases. We find that, independent of coverage, the HOMO energy of the linear chain phase is lower relative to the Fermi energy than that of the monomer phase, and in good agreement with values measured with ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy.
Computer Physics Communications
We summarize the molgw code that implements density-functional theory and many-body perturbation ... more We summarize the molgw code that implements density-functional theory and many-body perturbation theory in a Gaussian basis set. The code is dedicated to the calculation of the many-body self-energy within the GW approximation and the solution of the Bethe-Salpeter equation. These two types of calculations allow the user to evaluate physical quantities that can be compared to spectroscopic experiments. Quasiparticle energies, obtained through the calculation of the GW self-energy, can be compared to photoemission or transport experiments, and neutral excitation energies and oscillator strengths, obtained via solution of the Bethe-Salpeter equation, are measurable by optical absorption. The implementation choices outlined here have aimed at the accuracy and robustness of calculated quantities with respect to measurments. Furthermore, the algorithms implemented in molgw allow users to consider molecules or clusters containing up to 100 atoms with rather accurate basis sets, and to choose whether or not to apply the resolution-of-the-identity approximation. Finally, we demonstrate the parallelization efficacy of the molgw code over several hundreds of processors.
Physical Review Letters
We use a first-principles density functional theory approach to calculate the shift current and l... more We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current (∼100 µA/V 2), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective threedimensional electric polarization (∼1.9 C/m 2). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials
Journal of chemical theory and computation, Jan 14, 2016
Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technolo... more Charged excitations of the oligoacene family of molecules, relevant for astrophysics and technological applications, are widely studied and therefore provide an excellent system for benchmarking theoretical methods. In this work, we evaluate the performance of many-body perturbation theory within the GW approximation relative to new high-quality CCSD(T) reference data for charged excitations of the acenes. We compare GW calculations with a number of hybrid density functional theory starting points and with eigenvalue self-consistency. Special focus is given to elucidating the trend of GW-predicted excitations with molecule length increasing from benzene to hexacene. We find that GW calculations with starting points based on an optimally tuned range-separated hybrid (OTRSH) density functional and eigenvalue self-consistency can yield quantitative ionization potentials for the acenes. However, for larger acenes, the predicted electron affinities can deviate considerably from reference...
The Journal of chemical physics, Jan 21, 2017
The accurate prediction of singlet and triplet excitation energies is an area of intense research... more The accurate prediction of singlet and triplet excitation energies is an area of intense research of significant fundamental interest and critical for many applications. Most calculations of singlet and triplet energies use time-dependent density functional theory (TDDFT) in conjunction with an approximate exchange-correlation functional. In this work, we examine and critically assess an alternative method for predicting low-lying neutral excitations with similar computational cost, the ab initio Bethe-Salpeter equation (BSE) approach, and compare results against high-accuracy wavefunction-based methods. We consider singlet and triplet excitations of 27 prototypical organic molecules, including members of Thiel's set, the acene series, and several aromatic hydrocarbons exhibiting charge-transfer-like excitations. Analogous to its impact in TDDFT, we find that the Tamm-Dancoff approximation (TDA) overcomes triplet instabilities in the BSE approach, improving both triplet and sing...
Physical Review B, 2016
The Ruddlesden-Popper (RP) homologous series Srn+1TinO3n+1 provides a useful template for the stu... more The Ruddlesden-Popper (RP) homologous series Srn+1TinO3n+1 provides a useful template for the study and control of the effects of dimensionality and quantum confinement on the excited state properties of the complex oxide SrTiO3. We use ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach to calculate quasiparticle energies and absorption spectrum of Srn+1TinO3n+1 for n = 1 − 5 and ∞. Our computed direct and indirect optical gaps are in excellent agreement with spectroscopic measurements. The calculated optical spectra reproduce the main experimental features and reveal excitonic structure near the gap edge. We find that electron-hole interactions are important across the series, leading to significant exciton binding energies that increase for small n and reach a value of 330 meV for n = 1, a trend attributed to increased quantum confinement. We find that the lowest-energy singlet exciton of Sr2TiO4 (n = 1) localizes in the 2D plane defined by the TiO2 layer, and explain the origin of its localization.
Physical Review B, 2016
Molecular crystals are a prototypical class of van der Waals (vdW)-bound organic materials with e... more Molecular crystals are a prototypical class of van der Waals (vdW)-bound organic materials with excited state properties relevant for optoelectronics applications. Predicting the structure and excited state properties of molecular crystals presents a challenge for electronic structure theory, as standard approximations to density functional theory (DFT) do not capture long-range vdW dispersion interactions and do not yield excited-state properties. In this work, we use a combination of DFT including vdW forces-using both non-local correlation functionals and pair-wise correction methods-together with many-body perturbation theory (MBPT) to study the geometry and excited states, respectively, of the entire series of oligoacene crystals, from benzene to hexacene. We find that vdW methods can predict lattice constants within 1% of the experimental measurements, on par with the previously reported accuracy of pair-wise approximations for the same systems. We further find that excitation energies are sensitive to geometry, but if optimized geometries are used MBPT can yield excited state properties within a few tenths of an eV from experiment. We elucidate trends in MBPT-computed charged and neutral excitation energies across the acene series and discuss the role of common approximations used in MBPT.