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Papers by roberto dovesi

Research paper thumbnail of Characterization of the B-Center Defect in Diamond through the Vibrational Spectrum. A Quantum Mechanical Approach

The journal of physical chemistry. A, Jan 13, 2017

The B-center in diamond, that consists of a vacancy whose 4 first nearest neighbors are nitrogen ... more The B-center in diamond, that consists of a vacancy whose 4 first nearest neighbors are nitrogen atoms, has been investigated at the quantum mechanical level with an all electron Gaussian type basis set, hybrid functionals and the periodic supercell approach. In order to simulate various defect concentrations, four cubic supercells have been considered, containing (before the creation of the vacancy) 64, 216, 512 and 1000 atoms, respectively. Whereas the B-center does not affect the Raman spectrum of diamond, several intense peaks appear in the IR spectrum, that should permit to identify this defect. It turns out that, of the seven peaks proposed by Sutherland et al. (Nature, 174, pp. 901-904, 1954), located at 328, 780, 1003, 1171, 1332, 1372 and 1426 1/cm and frequently mentioned as fingerprints of the B center, the first one and the last three do not appear in the simulated spectrum at any concentration. The graphical animation of the modes (available at http://www.pmmp.jussieu.f...

Research paper thumbnail of Fcenter in LiF: A quantum mechanicalab initioinvestigation of the hyperfine interaction between the unpaired electron at the vacancy and its first seven neighbors

Physical Review B, 2001

The paramagnetic F center in LiF is investigated at a quantum mechanical level with the CRYSTAL98... more The paramagnetic F center in LiF is investigated at a quantum mechanical level with the CRYSTAL98 periodic code by using a supercell scheme. The isotropic and anisotropic components of the hyperfine coupling tensor describing the interaction between the unpaired electron and the nuclear spins up to the seventh nearest neighbors of the defect are computed by using two different Hamiltonians ͑Hartree-Fock and local density approximation͒, and turn out to compare reasonably well with electron paramagnetic resonance and electronnuclear double resonance data. A term of the hyperfine tensor, not yet available experimentally, is presented for future comparison. The defect wave function is analyzed in terms of the charge and spin density maps, band structure, and Mulliken population.

Research paper thumbnail of Calculation of the dynamic first electronic hyperpolarizabilityβ(−ωσ;ω1,ω2) of periodic systems. Theory, validation, and application to multi-layer MoS2

The Journal of Chemical Physics, 2015

Research paper thumbnail of The Raman spectrum of grossular garnet: a quantum mechanical simulation of wavenumbers and intensities

Journal of Raman Spectroscopy, 2014

Raman spectroscopy is a standard and powerful investigation technique for minerals, and garnet is... more Raman spectroscopy is a standard and powerful investigation technique for minerals, and garnet is one of the most observed and visible minerals, undoubtfully important both as a witness of our planet's evolution, and as a main component in many high-tech applications. This paper presents the Raman spectrum of grossular, the calcium-aluminium end-member of garnets (Ca 3 Al 2 Si 3 O 12), as computed by using an ab initio quantum-mechanical approach, an all-electron Gaussian-type basis set and the hybrid B3LYP functional. The wavenumbers of the 25 Raman active modes are in excellent agreement with the available experimental measurements, with the mean absolute difference being between 5 and 8 cm −1. The apparent disagreement between a few experimental vs calculated data can be easily justified through the analysis of the corresponding calculated peak intensities, which is very low in all of these cases. The intensities of the Raman active modes of grossular were calculated here for the first time, thanks to a recent implementation by some of the present authors that allows for accurate predictions of the Raman spectra of minerals. To the authors' knowledge, there are no tabulated data sets for Raman intensities of grossular, though qualitative information can be extracted from the published spectra. This study can then be considered as an accurate reference data set for grossular, other than a clear evidence that quantum-mechanical simulation is an actual tool to predict spectroscopic properties of minerals.

Research paper thumbnail of On the full exploitation of symmetry in periodic (as well as molecular) self-consistent-field ab initio calculations

The Journal of Chemical Physics, 2014

Use of symmetry can dramatically reduce the computational cost (running time and memory allocatio... more Use of symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for molecular and crystalline systems. Crucial for running time is symmetry exploitation in the evaluation of one-and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. Quantitative examples, referring to the implementation in the Crystal code, are given for high symmetry families of compounds such as carbon fullerenes and nanotubes.

Research paper thumbnail of Exploitation of symmetry in periodic Self-Consistent-Field ab initio calculations: application to large three-dimensional compounds

Science China Chemistry, 2014

Symmetry can dramatically reduce the computational cost (running time and memory allocation) of S... more Symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for crystalline systems. Crucial for running time is use of symmetry in the evaluation of one-and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. We here illustrate the effectiveness of this scheme, following recent advancements in the CRYSTAL code, concerning memory allocation and direct basis set transformation. Quantitative examples are given for large unit cell systems, such as zeolites (all-silica faujasite and silicalite MFI) and garnets (pyrope). It is shown that the full SCF of 3D systems containing up to 576 atoms and 11136 Atomic Orbitals in the cell can be run with a hybrid functional on a single core PC with 500 MB RAM in about 8 hours.

Research paper thumbnail of Ab Initio Quantum Simulation in Solid State Chemistry

B3LYP. In a few cases, HF is still preferred. 3. The basis set. Codes based on plane waves, local... more B3LYP. In a few cases, HF is still preferred. 3. The basis set. Codes based on plane waves, local functions, and mixed (local functions in atomic spheres, plane waves in the interstices) or numeric basis sets are available. 4. The overall computational scheme, in all its features, such as direct or reciprocal space representation, all-electron versus pseudo-potential

Research paper thumbnail of First-principles optical response of semiconductors and oxide materials

Physical Review B, 2011

The calculation of the optical gaps of a series of nonmagnetic direct and indirect semiconductors... more The calculation of the optical gaps of a series of nonmagnetic direct and indirect semiconductors and simple oxides is addressed using an all-electron perturbative method based on density-functional theory. Hybrid exchange, in both the Kohn-Sham spectrum and the perturbative response, is shown to be essential to achieve an accuracy comparable to experimental estimates for all systems studied, including those exhibiting excitonic transitions at the absorption edge. In agreement with existing evidence it is shown that a proper description of excitonic features relies crucially on the nonlocality of the response equations.

Research paper thumbnail of Ab initio vibrational spectra and dielectric properties of carbonates: magnesite, calcite and dolomite

Theoretical Chemistry Accounts, 2007

The equilibrium geometry, the Raman and IR vibrational spectra at the point, TO-LO splitting, IR ... more The equilibrium geometry, the Raman and IR vibrational spectra at the point, TO-LO splitting, IR intensities, Born and dielectric tensors of magnesite MgCO 3 , dolomite MgCa(CO 3) 2 and calcite CaCO 3 have been calculated with the periodic ab initio program CRYSTAL, by using an all-electron gaussian type basis set and the B3LYP hamiltonian. LO (longitudinaloptical) modes are computed by correcting the dynamical matrix through Born charges and high frequency

Research paper thumbnail of Analytical Hartree–Fock gradients with respect to the cell parameter: systems periodic in one and two dimensions

Theoretical Chemistry Accounts, 2005

Analytical Hartree-Fock gradients with respect to the cell parameter have been implemented in the... more Analytical Hartree-Fock gradients with respect to the cell parameter have been implemented in the electronic structure code CRYSTAL, for the case of one and two-dimensional periodicity. As in most molecular codes, Gaussian type orbitals are used to express the wavefunction. Examples demonstrate that the gradients have a good accuracy.

Research paper thumbnail of Beryllium Oxide Nanotubes and their Connection to the Flat Monolayer

The Journal of Physical Chemistry C, 2013

Single-walled zigzag Beryllium Oxide (BeO) nanotubes are simulated with an ab initio quantum chem... more Single-walled zigzag Beryllium Oxide (BeO) nanotubes are simulated with an ab initio quantum chemical method. The (n,0) family is investigated in the range from n = 8 (32 atoms in the unit cell and tube radius R = 3.4 Å) to n = 64 (256 atoms in the cell and R = 27.1 Å). The trend towards the hexagonal monolayer (h-BeO) in the limit of large tube radius R is explored for a variety of properties: rolling energy, elastic modulus, piezoelectric constant, vibration frequencies, infrared (IR) intensities, oscillator strengths, electronic and nuclear contributions to the polarizability tensor. Three sets of IR-active phonon bands are found in the spectrum. The first one lies in the 0-300 cm −1 frequency range and exhibits a very peculiar behavior: the vibration frequencies do tend regularly towards zero when R increases while their IR intensities do not; the nature of these normal modes is unveiled by establishing a connection between them and the elastic and piezoelectric constants of h-BeO. The second (680-730 cm −1) and third (1000-1200 cm −1) sets tend regularly, but with quite different slope, to the optical modes of the h-BeO layer. The vibrational contribution of these modes to the two components (parallel and perpendicular) of the polarizability tensor is also discussed. Simulations are performed using the CRYSTAL program which fully exploits the rich symmetry of this class of one-dimensional periodic systems: 4n symmetry operators for the general (n,0) tube.

Research paper thumbnail of The First and Second Static Electronic Hyperpolarizabilities of Zigzag Boron Nitride Nanotubes. An ab Initio Approach through the Coupled Perturbed Kohn–Sham Scheme

The Journal of Physical Chemistry A, 2011

Research paper thumbnail of Elastic properties of six silicate garnet end members from accurate ab initio simulations

Physics and Chemistry of Minerals, 2013

The elastic properties of six silicate garnet end-members, among the most important rock-forming ... more The elastic properties of six silicate garnet end-members, among the most important rock-forming minerals, are here investigated for the first time via accurate ab initio theoretical simulations. The Crystal program is used which works within periodic boundary conditions and allows for all-electron basis sets to be adopted. From the computed elastic tensor, Christoffel's equation is solved along a set of crystallographic directions in order to fully characterize the seismic wave velocity anisotropy in such materials. Polycrystalline isotropic aggregate elastic properties are derived from the computed single-crystal data via the Voigt-Reuss-Hill averaging procedure. Transferability of the elastic properties from end-members to their solid solutions with different chemical compositions is also addressed.

Research paper thumbnail of Polarizability and hyperpolarizability of BN zigzag nanotubes calculated by the coupled perturbed Kohn-Sham scheme

Physical Review B, 2011

Linear and nonlinear electric dipole susceptibilities are evaluated for infinite periodic zigzag ... more Linear and nonlinear electric dipole susceptibilities are evaluated for infinite periodic zigzag BN nanotubes utilizing primarily the coupled perturbed Kohn-Sham scheme recently implemented in the CRYSTAL code. The effect of different functionals, basis set, and computational parameters is examined. Most of the calculations are done at the B3LYP/6-31G* level. For electronic linear polarizabilities, substantial differences compared to the uncoupled sum-overstates scheme are found. Much larger radii were considered than in earlier studies, thereby permitting accurate comparison with corresponding properties of the hexagonal monolayer. In addition, we confirmed the dielectric shell model for the linear polarizability, but with a significantly different shell thickness than previously thought. Vibrational (ionic) contributions to the nonlinear susceptibilities are calculated. In doing so, the finite-field-nuclear-relaxation (FF-NR) method was employed for the transverse components of the (6,0), (9,0), and (12,0) nanotubes. Aside from being computationally more efficient than other procedures, this method includes anharmonicity effects through first order and, as shown, is readily applied to key dynamic as well as static properties (and yields the static linear polarizability as well). Our calculated nonlinear vibrational susceptibilities sometimes exceed, or even greatly exceed, the corresponding static electronic susceptibility. In such cases, the relative magnitude of the vibrational contribution grows substantially with tube radius over the range considered. Future plans include extending these FF-NR calculations to large nanotubes and to the longitudinal (periodic) direction as well.

Research paper thumbnail of Superexchange interaction in K 2 NiF 4 : an ab initio Hartree-Fock study

Journal of Physics: Condensed Matter, 1995

The ground-state electronic structures of the ferromagnetic and antiferromagnetic phases of K2NiF... more The ground-state electronic structures of the ferromagnetic and antiferromagnetic phases of K2NiF4 have been investigated using the ab initio periodic Hartree-Fock approach. The system is a wide-gap insulator. The antiferromagnetic phase is more stable than the ferromagnetic phase by 0.0216 eV per Ni pair, which is almost exactly two thirds of that found in KNiF3, in agreement with the hypothesis

Research paper thumbnail of The calculation of static polarizabilities of 1-3D periodic compounds. the implementation in the crystal code

Journal of Computational Chemistry, 2008

The Coupled Perturbed Hartree-Fock (CPHF) scheme has been implemented in the CRYSTAL06 program, t... more The Coupled Perturbed Hartree-Fock (CPHF) scheme has been implemented in the CRYSTAL06 program, that uses a gaussian type basis set, for systems periodic in 1D (polymers), 2D (slabs), 3D (crystals) and, as a limiting case, 0D (molecules), which enables comparison with molecular codes. CPHF is applied to the calculation of the polarizability α of LiF in different aggregation states: finite and infinite chains, slabs, and cubic crystal. Correctness of the computational scheme for the various dimensionalities and its numerical efficiency are confirmed by the correct trend of α: α for a finite linear chain containing N LiF units with large N tends to the value for the infinite chain, N parallel chains give the slab value when N is sufficiently large, and N superimposed slabs tend to the bulk value. CPHF results compare well with those obtained with a saw-tooth potential approach, previously implemented in CRYSTAL. High numerical accuracy can easily be achieved at relatively low cost, with the same kind of dependence on the computational parameters as for the SCF cycle. Overall, the cost of one component of the dielectric tensor is roughly the same as for the SCF cycle, and it is dominated by the calculation of two-electron four-center integrals.

Research paper thumbnail of Performance of six functionals (LDA, PBE, PBESOL, B3LYP, PBE0, and WC1LYP) in the simulation of vibrational and dielectric properties of crystalline compounds. The case of forsterite Mg2SiO4

Journal of Computational Chemistry, 2011

Research paper thumbnail of A new massively parallel version of CRYSTAL for large systems on high performance computing architectures

Journal of Computational Chemistry, 2012

Fully ab initio treatment of complex solid systems needs computational software which is able to ... more Fully ab initio treatment of complex solid systems needs computational software which is able to efficiently take advantage of the growing power of high performance computing (HPC) architectures. Recent improvements in CRYSTAL, a periodic ab initio code that uses a Gaussian basis set, allows treatment of very large unit cells for crystalline systems on HPC architectures with high parallel efficiency in terms of running time and memory requirements. The latter is a crucial point, due to the trend toward architectures relying on a very high number of cores with associated relatively low memory availability. An exhaustive performance analysis shows that density functional calculations, based on a hybrid functional, of low-symmetry systems containing up to 100,000 atomic orbitals and 8000 atoms are feasible on the most advanced HPC architectures available to European researchers today, using thousands of processors. V

Research paper thumbnail of The Raman spectrum of CaCO3polymorphs calcite and aragonite: A combined experimental and computational study

The Journal of Chemical Physics, 2014

Powder and single crystal Raman spectra of the two most common phases of calcium carbonate are ca... more Powder and single crystal Raman spectra of the two most common phases of calcium carbonate are calculated with ab initio techniques (using a "hybrid" functional and a gaussian-type basis set) and measured both at 80 K and room temperature. Frequencies of the Raman modes are in very good agreement between calculations and experiments: the mean absolute deviation at 80 K is 4 and 8 cm −1 for calcite and aragonite, respectively. As regards intensities, the agreement is in general good, although the computed values overestimate the measured ones in many cases. The combined analysis permits to identify almost all the fundamental experimental Raman peaks of the two compounds, with the exception of either modes with zero computed intensity or modes overlapping with more intense peaks. Additional peaks have been identified in both calcite and aragonite, which have been assigned to 18 O satellite modes or overtones. The agreement between the computed and measured spectra is quite satisfactory; in particular, simulation permits to clearly distinguish between calcite and aragonite in the case of powder spectra, and among different polarization directions of each compound in the case of single crystal spectra.

Research paper thumbnail of The calculation of the static first and second susceptibilities of crystalline urea: A comparison of Hartree–Fock and density functional theory results obtained with the periodic coupled perturbed Hartree–Fock/Kohn–Sham scheme

The Journal of Chemical Physics, 2009

The static polarizability alpha and first hyperpolarizability beta tensors of crystalline urea an... more The static polarizability alpha and first hyperpolarizability beta tensors of crystalline urea and the corresponding first-(chi((1))) and second-(chi((2))) susceptibilities are calculated and compared to the same quantities obtained for the molecule by using the same code (a development version of CRYSTAL), basis set, and level of theory. In order to separate geometrical and solid state effects, two geometries are considered for the molecule in its planar conformation: (i) as cut out from the bulk structure and (ii) fully optimized. First, the effect of basis sets on computed properties is explored at the B3LYP level by employing basis sets of increasing complexity, from 6-31G(d,p) to 6-311G(2df,2pd) (Pople's family) and from DZP to QZVPPP (Thakkar/Ahlrichs/Dunning's family) on alpha and beta for both the molecule and the bulk. Then, five different levels of theory, namely, SVWN (local density approximation), PBE (generalized gradient approximation), PBE0 and B3LYP (hybrid), and Hartree-Fock are compared in combination with a TZPP basis set. Present results show that hybrid methods, in particular, B3LYP, are remarkably successful in predicting correctly both the first and second susceptibilities of urea bulk when combined at least with a triple-zeta quality basis set containing a double set of polarization functions. It is also shown that diffuse functions that are needed for molecular calculations are less crucial for the crystalline structure, as expected. Indeed, B3LYP/TZPP computed chi((1)) and chi((2)) tensor components (chi(aa) ((1))=1.107, chi(cc) ((1))=1.459, and chi((2))=-0.93 a.u.) are in very good agreement with experimental values. At variance with respect to previous periodic ab initio calculations, but in agreement with recent supermolecular results, the negative sign of chi((2)) is confirmed. Overall, static linear and nonlinear optical properties such as dielectric constants, refractive, and birefringence indices and second-harmonic generation coefficient of crystalline urea are very well reproduced by present calculations.

Research paper thumbnail of Characterization of the B-Center Defect in Diamond through the Vibrational Spectrum. A Quantum Mechanical Approach

The journal of physical chemistry. A, Jan 13, 2017

The B-center in diamond, that consists of a vacancy whose 4 first nearest neighbors are nitrogen ... more The B-center in diamond, that consists of a vacancy whose 4 first nearest neighbors are nitrogen atoms, has been investigated at the quantum mechanical level with an all electron Gaussian type basis set, hybrid functionals and the periodic supercell approach. In order to simulate various defect concentrations, four cubic supercells have been considered, containing (before the creation of the vacancy) 64, 216, 512 and 1000 atoms, respectively. Whereas the B-center does not affect the Raman spectrum of diamond, several intense peaks appear in the IR spectrum, that should permit to identify this defect. It turns out that, of the seven peaks proposed by Sutherland et al. (Nature, 174, pp. 901-904, 1954), located at 328, 780, 1003, 1171, 1332, 1372 and 1426 1/cm and frequently mentioned as fingerprints of the B center, the first one and the last three do not appear in the simulated spectrum at any concentration. The graphical animation of the modes (available at http://www.pmmp.jussieu.f...

Research paper thumbnail of Fcenter in LiF: A quantum mechanicalab initioinvestigation of the hyperfine interaction between the unpaired electron at the vacancy and its first seven neighbors

Physical Review B, 2001

The paramagnetic F center in LiF is investigated at a quantum mechanical level with the CRYSTAL98... more The paramagnetic F center in LiF is investigated at a quantum mechanical level with the CRYSTAL98 periodic code by using a supercell scheme. The isotropic and anisotropic components of the hyperfine coupling tensor describing the interaction between the unpaired electron and the nuclear spins up to the seventh nearest neighbors of the defect are computed by using two different Hamiltonians ͑Hartree-Fock and local density approximation͒, and turn out to compare reasonably well with electron paramagnetic resonance and electronnuclear double resonance data. A term of the hyperfine tensor, not yet available experimentally, is presented for future comparison. The defect wave function is analyzed in terms of the charge and spin density maps, band structure, and Mulliken population.

Research paper thumbnail of Calculation of the dynamic first electronic hyperpolarizabilityβ(−ωσ;ω1,ω2) of periodic systems. Theory, validation, and application to multi-layer MoS2

The Journal of Chemical Physics, 2015

Research paper thumbnail of The Raman spectrum of grossular garnet: a quantum mechanical simulation of wavenumbers and intensities

Journal of Raman Spectroscopy, 2014

Raman spectroscopy is a standard and powerful investigation technique for minerals, and garnet is... more Raman spectroscopy is a standard and powerful investigation technique for minerals, and garnet is one of the most observed and visible minerals, undoubtfully important both as a witness of our planet's evolution, and as a main component in many high-tech applications. This paper presents the Raman spectrum of grossular, the calcium-aluminium end-member of garnets (Ca 3 Al 2 Si 3 O 12), as computed by using an ab initio quantum-mechanical approach, an all-electron Gaussian-type basis set and the hybrid B3LYP functional. The wavenumbers of the 25 Raman active modes are in excellent agreement with the available experimental measurements, with the mean absolute difference being between 5 and 8 cm −1. The apparent disagreement between a few experimental vs calculated data can be easily justified through the analysis of the corresponding calculated peak intensities, which is very low in all of these cases. The intensities of the Raman active modes of grossular were calculated here for the first time, thanks to a recent implementation by some of the present authors that allows for accurate predictions of the Raman spectra of minerals. To the authors' knowledge, there are no tabulated data sets for Raman intensities of grossular, though qualitative information can be extracted from the published spectra. This study can then be considered as an accurate reference data set for grossular, other than a clear evidence that quantum-mechanical simulation is an actual tool to predict spectroscopic properties of minerals.

Research paper thumbnail of On the full exploitation of symmetry in periodic (as well as molecular) self-consistent-field ab initio calculations

The Journal of Chemical Physics, 2014

Use of symmetry can dramatically reduce the computational cost (running time and memory allocatio... more Use of symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for molecular and crystalline systems. Crucial for running time is symmetry exploitation in the evaluation of one-and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. Quantitative examples, referring to the implementation in the Crystal code, are given for high symmetry families of compounds such as carbon fullerenes and nanotubes.

Research paper thumbnail of Exploitation of symmetry in periodic Self-Consistent-Field ab initio calculations: application to large three-dimensional compounds

Science China Chemistry, 2014

Symmetry can dramatically reduce the computational cost (running time and memory allocation) of S... more Symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for crystalline systems. Crucial for running time is use of symmetry in the evaluation of one-and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. We here illustrate the effectiveness of this scheme, following recent advancements in the CRYSTAL code, concerning memory allocation and direct basis set transformation. Quantitative examples are given for large unit cell systems, such as zeolites (all-silica faujasite and silicalite MFI) and garnets (pyrope). It is shown that the full SCF of 3D systems containing up to 576 atoms and 11136 Atomic Orbitals in the cell can be run with a hybrid functional on a single core PC with 500 MB RAM in about 8 hours.

Research paper thumbnail of Ab Initio Quantum Simulation in Solid State Chemistry

B3LYP. In a few cases, HF is still preferred. 3. The basis set. Codes based on plane waves, local... more B3LYP. In a few cases, HF is still preferred. 3. The basis set. Codes based on plane waves, local functions, and mixed (local functions in atomic spheres, plane waves in the interstices) or numeric basis sets are available. 4. The overall computational scheme, in all its features, such as direct or reciprocal space representation, all-electron versus pseudo-potential

Research paper thumbnail of First-principles optical response of semiconductors and oxide materials

Physical Review B, 2011

The calculation of the optical gaps of a series of nonmagnetic direct and indirect semiconductors... more The calculation of the optical gaps of a series of nonmagnetic direct and indirect semiconductors and simple oxides is addressed using an all-electron perturbative method based on density-functional theory. Hybrid exchange, in both the Kohn-Sham spectrum and the perturbative response, is shown to be essential to achieve an accuracy comparable to experimental estimates for all systems studied, including those exhibiting excitonic transitions at the absorption edge. In agreement with existing evidence it is shown that a proper description of excitonic features relies crucially on the nonlocality of the response equations.

Research paper thumbnail of Ab initio vibrational spectra and dielectric properties of carbonates: magnesite, calcite and dolomite

Theoretical Chemistry Accounts, 2007

The equilibrium geometry, the Raman and IR vibrational spectra at the point, TO-LO splitting, IR ... more The equilibrium geometry, the Raman and IR vibrational spectra at the point, TO-LO splitting, IR intensities, Born and dielectric tensors of magnesite MgCO 3 , dolomite MgCa(CO 3) 2 and calcite CaCO 3 have been calculated with the periodic ab initio program CRYSTAL, by using an all-electron gaussian type basis set and the B3LYP hamiltonian. LO (longitudinaloptical) modes are computed by correcting the dynamical matrix through Born charges and high frequency

Research paper thumbnail of Analytical Hartree–Fock gradients with respect to the cell parameter: systems periodic in one and two dimensions

Theoretical Chemistry Accounts, 2005

Analytical Hartree-Fock gradients with respect to the cell parameter have been implemented in the... more Analytical Hartree-Fock gradients with respect to the cell parameter have been implemented in the electronic structure code CRYSTAL, for the case of one and two-dimensional periodicity. As in most molecular codes, Gaussian type orbitals are used to express the wavefunction. Examples demonstrate that the gradients have a good accuracy.

Research paper thumbnail of Beryllium Oxide Nanotubes and their Connection to the Flat Monolayer

The Journal of Physical Chemistry C, 2013

Single-walled zigzag Beryllium Oxide (BeO) nanotubes are simulated with an ab initio quantum chem... more Single-walled zigzag Beryllium Oxide (BeO) nanotubes are simulated with an ab initio quantum chemical method. The (n,0) family is investigated in the range from n = 8 (32 atoms in the unit cell and tube radius R = 3.4 Å) to n = 64 (256 atoms in the cell and R = 27.1 Å). The trend towards the hexagonal monolayer (h-BeO) in the limit of large tube radius R is explored for a variety of properties: rolling energy, elastic modulus, piezoelectric constant, vibration frequencies, infrared (IR) intensities, oscillator strengths, electronic and nuclear contributions to the polarizability tensor. Three sets of IR-active phonon bands are found in the spectrum. The first one lies in the 0-300 cm −1 frequency range and exhibits a very peculiar behavior: the vibration frequencies do tend regularly towards zero when R increases while their IR intensities do not; the nature of these normal modes is unveiled by establishing a connection between them and the elastic and piezoelectric constants of h-BeO. The second (680-730 cm −1) and third (1000-1200 cm −1) sets tend regularly, but with quite different slope, to the optical modes of the h-BeO layer. The vibrational contribution of these modes to the two components (parallel and perpendicular) of the polarizability tensor is also discussed. Simulations are performed using the CRYSTAL program which fully exploits the rich symmetry of this class of one-dimensional periodic systems: 4n symmetry operators for the general (n,0) tube.

Research paper thumbnail of The First and Second Static Electronic Hyperpolarizabilities of Zigzag Boron Nitride Nanotubes. An ab Initio Approach through the Coupled Perturbed Kohn–Sham Scheme

The Journal of Physical Chemistry A, 2011

Research paper thumbnail of Elastic properties of six silicate garnet end members from accurate ab initio simulations

Physics and Chemistry of Minerals, 2013

The elastic properties of six silicate garnet end-members, among the most important rock-forming ... more The elastic properties of six silicate garnet end-members, among the most important rock-forming minerals, are here investigated for the first time via accurate ab initio theoretical simulations. The Crystal program is used which works within periodic boundary conditions and allows for all-electron basis sets to be adopted. From the computed elastic tensor, Christoffel's equation is solved along a set of crystallographic directions in order to fully characterize the seismic wave velocity anisotropy in such materials. Polycrystalline isotropic aggregate elastic properties are derived from the computed single-crystal data via the Voigt-Reuss-Hill averaging procedure. Transferability of the elastic properties from end-members to their solid solutions with different chemical compositions is also addressed.

Research paper thumbnail of Polarizability and hyperpolarizability of BN zigzag nanotubes calculated by the coupled perturbed Kohn-Sham scheme

Physical Review B, 2011

Linear and nonlinear electric dipole susceptibilities are evaluated for infinite periodic zigzag ... more Linear and nonlinear electric dipole susceptibilities are evaluated for infinite periodic zigzag BN nanotubes utilizing primarily the coupled perturbed Kohn-Sham scheme recently implemented in the CRYSTAL code. The effect of different functionals, basis set, and computational parameters is examined. Most of the calculations are done at the B3LYP/6-31G* level. For electronic linear polarizabilities, substantial differences compared to the uncoupled sum-overstates scheme are found. Much larger radii were considered than in earlier studies, thereby permitting accurate comparison with corresponding properties of the hexagonal monolayer. In addition, we confirmed the dielectric shell model for the linear polarizability, but with a significantly different shell thickness than previously thought. Vibrational (ionic) contributions to the nonlinear susceptibilities are calculated. In doing so, the finite-field-nuclear-relaxation (FF-NR) method was employed for the transverse components of the (6,0), (9,0), and (12,0) nanotubes. Aside from being computationally more efficient than other procedures, this method includes anharmonicity effects through first order and, as shown, is readily applied to key dynamic as well as static properties (and yields the static linear polarizability as well). Our calculated nonlinear vibrational susceptibilities sometimes exceed, or even greatly exceed, the corresponding static electronic susceptibility. In such cases, the relative magnitude of the vibrational contribution grows substantially with tube radius over the range considered. Future plans include extending these FF-NR calculations to large nanotubes and to the longitudinal (periodic) direction as well.

Research paper thumbnail of Superexchange interaction in K 2 NiF 4 : an ab initio Hartree-Fock study

Journal of Physics: Condensed Matter, 1995

The ground-state electronic structures of the ferromagnetic and antiferromagnetic phases of K2NiF... more The ground-state electronic structures of the ferromagnetic and antiferromagnetic phases of K2NiF4 have been investigated using the ab initio periodic Hartree-Fock approach. The system is a wide-gap insulator. The antiferromagnetic phase is more stable than the ferromagnetic phase by 0.0216 eV per Ni pair, which is almost exactly two thirds of that found in KNiF3, in agreement with the hypothesis

Research paper thumbnail of The calculation of static polarizabilities of 1-3D periodic compounds. the implementation in the crystal code

Journal of Computational Chemistry, 2008

The Coupled Perturbed Hartree-Fock (CPHF) scheme has been implemented in the CRYSTAL06 program, t... more The Coupled Perturbed Hartree-Fock (CPHF) scheme has been implemented in the CRYSTAL06 program, that uses a gaussian type basis set, for systems periodic in 1D (polymers), 2D (slabs), 3D (crystals) and, as a limiting case, 0D (molecules), which enables comparison with molecular codes. CPHF is applied to the calculation of the polarizability α of LiF in different aggregation states: finite and infinite chains, slabs, and cubic crystal. Correctness of the computational scheme for the various dimensionalities and its numerical efficiency are confirmed by the correct trend of α: α for a finite linear chain containing N LiF units with large N tends to the value for the infinite chain, N parallel chains give the slab value when N is sufficiently large, and N superimposed slabs tend to the bulk value. CPHF results compare well with those obtained with a saw-tooth potential approach, previously implemented in CRYSTAL. High numerical accuracy can easily be achieved at relatively low cost, with the same kind of dependence on the computational parameters as for the SCF cycle. Overall, the cost of one component of the dielectric tensor is roughly the same as for the SCF cycle, and it is dominated by the calculation of two-electron four-center integrals.

Research paper thumbnail of Performance of six functionals (LDA, PBE, PBESOL, B3LYP, PBE0, and WC1LYP) in the simulation of vibrational and dielectric properties of crystalline compounds. The case of forsterite Mg2SiO4

Journal of Computational Chemistry, 2011

Research paper thumbnail of A new massively parallel version of CRYSTAL for large systems on high performance computing architectures

Journal of Computational Chemistry, 2012

Fully ab initio treatment of complex solid systems needs computational software which is able to ... more Fully ab initio treatment of complex solid systems needs computational software which is able to efficiently take advantage of the growing power of high performance computing (HPC) architectures. Recent improvements in CRYSTAL, a periodic ab initio code that uses a Gaussian basis set, allows treatment of very large unit cells for crystalline systems on HPC architectures with high parallel efficiency in terms of running time and memory requirements. The latter is a crucial point, due to the trend toward architectures relying on a very high number of cores with associated relatively low memory availability. An exhaustive performance analysis shows that density functional calculations, based on a hybrid functional, of low-symmetry systems containing up to 100,000 atomic orbitals and 8000 atoms are feasible on the most advanced HPC architectures available to European researchers today, using thousands of processors. V

Research paper thumbnail of The Raman spectrum of CaCO3polymorphs calcite and aragonite: A combined experimental and computational study

The Journal of Chemical Physics, 2014

Powder and single crystal Raman spectra of the two most common phases of calcium carbonate are ca... more Powder and single crystal Raman spectra of the two most common phases of calcium carbonate are calculated with ab initio techniques (using a "hybrid" functional and a gaussian-type basis set) and measured both at 80 K and room temperature. Frequencies of the Raman modes are in very good agreement between calculations and experiments: the mean absolute deviation at 80 K is 4 and 8 cm −1 for calcite and aragonite, respectively. As regards intensities, the agreement is in general good, although the computed values overestimate the measured ones in many cases. The combined analysis permits to identify almost all the fundamental experimental Raman peaks of the two compounds, with the exception of either modes with zero computed intensity or modes overlapping with more intense peaks. Additional peaks have been identified in both calcite and aragonite, which have been assigned to 18 O satellite modes or overtones. The agreement between the computed and measured spectra is quite satisfactory; in particular, simulation permits to clearly distinguish between calcite and aragonite in the case of powder spectra, and among different polarization directions of each compound in the case of single crystal spectra.

Research paper thumbnail of The calculation of the static first and second susceptibilities of crystalline urea: A comparison of Hartree–Fock and density functional theory results obtained with the periodic coupled perturbed Hartree–Fock/Kohn–Sham scheme

The Journal of Chemical Physics, 2009

The static polarizability alpha and first hyperpolarizability beta tensors of crystalline urea an... more The static polarizability alpha and first hyperpolarizability beta tensors of crystalline urea and the corresponding first-(chi((1))) and second-(chi((2))) susceptibilities are calculated and compared to the same quantities obtained for the molecule by using the same code (a development version of CRYSTAL), basis set, and level of theory. In order to separate geometrical and solid state effects, two geometries are considered for the molecule in its planar conformation: (i) as cut out from the bulk structure and (ii) fully optimized. First, the effect of basis sets on computed properties is explored at the B3LYP level by employing basis sets of increasing complexity, from 6-31G(d,p) to 6-311G(2df,2pd) (Pople's family) and from DZP to QZVPPP (Thakkar/Ahlrichs/Dunning's family) on alpha and beta for both the molecule and the bulk. Then, five different levels of theory, namely, SVWN (local density approximation), PBE (generalized gradient approximation), PBE0 and B3LYP (hybrid), and Hartree-Fock are compared in combination with a TZPP basis set. Present results show that hybrid methods, in particular, B3LYP, are remarkably successful in predicting correctly both the first and second susceptibilities of urea bulk when combined at least with a triple-zeta quality basis set containing a double set of polarization functions. It is also shown that diffuse functions that are needed for molecular calculations are less crucial for the crystalline structure, as expected. Indeed, B3LYP/TZPP computed chi((1)) and chi((2)) tensor components (chi(aa) ((1))=1.107, chi(cc) ((1))=1.459, and chi((2))=-0.93 a.u.) are in very good agreement with experimental values. At variance with respect to previous periodic ab initio calculations, but in agreement with recent supermolecular results, the negative sign of chi((2)) is confirmed. Overall, static linear and nonlinear optical properties such as dielectric constants, refractive, and birefringence indices and second-harmonic generation coefficient of crystalline urea are very well reproduced by present calculations.