On-the-fly ab initio semiclassical evaluation of time-resolved electronic spectra (original) (raw)
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Simulating vibrationally resolved electronic spectra of anharmonic systems, especially those involving double-well potential energy surfaces, often requires expensive quantum dynamics methods. Here, we explore the applicability and limitations of the recently proposed single-Hessian thawed Gaussian approximation for the simulation of spectra of systems with double-well potentials, including 1,2,4,5-tetrafluorobenzene, ammonia, phosphine, and arsine. This semiclassical wavepacket approach is shown to be more robust and to provide more accurate spectra than the conventional harmonic approximation. At the same time, the method is efficient and requires only a single 1
Chemical Physics
Evaluation of symmetry-forbidden or weakly-allowed vibronic spectra requires treating the transition dipole moment beyond the Condon approximation. To include vibronic spectral features not captured in the global harmonic models, we have recently implemented an on-the-fly ab initio extended thawed Gaussian approximation, where the propagated wavepacket is a Gaussian multiplied by a linear polynomial. To include more anharmonic effects, here we represent the initial wavepacket by a superposition of three independent Gaussian wavepackets-one for the Condon term and two displaced Gaussians for the Herzberg-Teller part. Application of this ab initio "three thawed Gaussians approximation" to vibrationally resolved electronic spectra of the phenyl radical and benzene shows a clear improvement over the global harmonic and Condon approximations. The orientational averaging of spectra, the relation between the gradient of the transition dipole moment and nonadiabatic coupling vectors, and the details of the extended and three thawed Gaussians approximation are discussed. i.e., the second-order expansion of the potential about the center of the wavepacket. Such a wavepacket, however, is wider than a simple Gaussian wavepacket, which raises the question whether more than a single guiding classical trajectory should be used to correctly account for the anharmonicity of the potential energy surface.
International Journal of Quantum Chemistry, 2010
Ab initio methods for electronic structure of molecules have reached a satisfactory accuracy for calculation of static properties, but remain too expensive for quantum dynamical calculations. We propose an efficient semiclassical method for evaluating the accuracy of a lower level quantum dynamics, as compared to a higher level quantum dynamics, without having to perform any quantum dynamics. The method is based on dephasing representation of quantum fidelity and its feasibility is demonstrated on the photodissociation dynamics of CO2. We suggest how to implement the method in existing molecular dynamics codes and describe a simple test of its applicability.
2004
Purpose With the trend towards needing information about chemistry at conditions significantly different from 298K and 1 atm., methods need to be developed to generate and interpret this data. This demand for information about chemistry at extreme conditions comes from many fields. The study of atmospheric chemistry requires knowledge of unusual species that are formed when molecules are exposed to ultraviolet radiation. Studying of energetic materials requires knowledge of the thermochemical and structural properties of a myriad of chemical species under a wide range of temperatures. Basic scientific understanding of the very nature of a chemical bond requires detailed information. Studying these problems computationally requires multiple capabilities. The methodology used must provide both high accuracy and computational efficiency. Studying extreme chemistry also suffers from all the challenges of studying chemistry under non-extreme conditions. Therefore, either a new method must be developed or an old method must be applied in an innovative way. The approach The method we have chosen to use is path integral Monte Carlo (PIMC) for the nuclear degrees of freedom and ab initio electronic structure methods for the electronic degrees of freedom. PIMC and ab initio electronic structure are methods of treating the quantum nature of particles. These methods have been chosen, because an accurate treatment requires treating both the electrons and the nuclei as quantum particles. We developed new "projected" methods that reduce the computational demands. These methods along with PIMC in general are described in two Journal of Chemical Physics articles (UCRL-JC-144960 and UCRL-JC-147423). This methodology was implemented into a PIMC code developed as part of this LDRD. The code was parallelized in order to utilize the computational resources of LLNL. Technical accomplishments By coupling quantum treatment of the electrons via ab initio method to the quantum motion of the nuclei with PIMC, a method that is able to produce accurate results is obtained. This level of accuracy is necessary under extreme conditions, because traditional "empirical" corrections are not applicable under such conditions. Because the electronic structure is tightly coupled to the motion of the nuclei, ab initio methods must be used to treat the electrons. These methods have explicit molecular orbital dependence are not parameterized to simply reproduce an equilibrium geometry. This method can calculate structural information, energetics, and heat capacities. The thermochemical results are described in detail in a Journal of Chemical Physics article (UCRL-JC-149046). Conclusion A theoretical method must provide the level of accuracy needed to answer the question of interest; otherwise, the results could be potentially misleading. This methodology used in this LDRD provided both structural and energetic information. The method used provides a fully quantum approach to calculation properties of molecules under extreme conditions. The software to perform these calculations is written and is integrated with a variety of electronic structure codes (GAMESS and Fireball). The code incorporates new computer time saving methodologies. This method calculates the properties needed, such as energy and heat capacity. The code continues to be used after the completion of the LDRD.
We present an efficient quantum algorithm for beyond-Born-Oppenheimer molecular energy computations. Our approach combines the quantum full configuration interaction method with the nuclear orbital plus molecular orbital (NOMO) method. We give the details of the algorithm and demonstrate its performance by classical simulations. Two isotopomers of the hydrogen molecule (H2, HT) were chosen as representative examples and calculations of the lowest rotationless vibrational transition energies were simulated. ----------- Next version (in preparation): The new algorithm for the evolution operator (7) part corresponding to bosons in the novel "compact boson" mapping will be added. This decrease the computational cost a bit. ---------- The latest version is to be published in the International Journal of Quantum Chemistry (IJQC): L., Veis, J., Višňák, H., Nishizawa, H., Nakai, Jiří, Pittner. Int. J. Quantum Chem. 2016, DOI: 10.1002/qua.25176 http://onlinelibrary.wiley.com/wol1/doi/10.1002/qua.25176/abstract
Quantitative quantum chemistry
2008
We review the current status of quantum chemistry as a predictive tool of chemistry and molecular physics, capable of providing highly accurate, quantitative data about molecular systems. We begin by reviewing wave-function based electronic-structure theory, emphasizing the N-electron hierarchy of coupled-cluster theory and the one-electron hierarchy of correlation-consistent basis sets. Following a discussion of the slow basis-set convergence of dynamical correlation and basis-set extrapolations, we consider the methods of explicit correlation, from the early work of Hylleraas in the 1920s to the latest developments in such methods, capable of yielding high-accuracy results in medium-sized basis sets. Next, we consider the small corrections to the electronic energy (high-order virtual excitations, vibrational, relativistic, and diagonal Born-Oppenheimer corrections) needed for high accuracy and conclude with a review of the composite methods and computational protocols of electronic-structure theory.
Journal of Chemical Theory and Computation, 2006
In a recent publication, we introduced a computational approach to treat the simultaneous dynamics of electrons and nuclei. The method is based on a synergy between quantum wave packet dynamics and ab initio molecular dynamics. Atom-centered density-matrix propagation or Born-Oppenheimer dynamics can be used to perform ab initio dynamics. In this paper, wave packet dynamics is conducted using a three-dimensional direct product implementation of the distributed approximating functional free-propagator. A fundamental computational difficulty in this approach is that the interaction potential between the two components of the methodology needs to be calculated frequently. Here, we overcome this problem through the use of a time-dependent deterministic sampling measure that predicts, at every step of the dynamics, regions of the potential which are important. The algorithm, when combined with an on-the-fly interpolation scheme, allows us to determine the quantum dynamical interaction potential and gradients at every dynamics step in an extremely efficient manner. Numerical demonstrations of our sampling algorithm are provided through several examples arranged in a cascading level of complexity. Starting from a simple one-dimensional quantum dynamical treatment of the shared proton in [Cl-H-Cl]and [CH 3 -H-Cl]along with simultaneous dynamical treatment of the electrons and classical nuclei, through a complete three-dimensional treatment of the shared proton in [Cl-H-Cl]as well as treatment of a hydrogen atom undergoing donor-acceptor transitions in the biological enzyme, soybean lipoxygenase-1 (SLO-1), we benchmark the algorithm thoroughly. Apart from computing various error estimates, we also compare vibrational density of states, inclusive of full quantum effects from the shared proton, using a novel unified velocity-velocity, flux-flux autocorrelation function. In all cases, the potential-adapted, time-dependent sampling procedure is seen to improve the computational scheme tremendously (by orders of magnitude) with minimal loss of accuracy.
Physical Chemistry Chemical Physics, 2011
The aim of the study was to explore the limits of initio methods towards the description of excited vibrational levels up to the dissociation limit for molecules having more than two electrons. To this end a high level ab initio potential energy function was constructed for the four-electron LiH molecule in order to accurately predict a complete set of bound vibrational levels corresponding to the electronic ground state. It was composed from: a) an ab initio non-relativistic potential obtained at the MR-CISD level including size-extensivity corrections and quintuple-sextuple ζ extrapolation of the basis, b) MVD relativistic corrections obtained at icMR-CISD/cc-pwCV5Z level, and c) DBOC obtained at the MR-CISD/cc-pwCVTZ level. Finally, the importance of non-adiabatic effects was also tested by using atomic masses in the vibrational kinetic energy operator. The calculated vibrational levels were compared with those obtained from experimental data [J.A. Coxon and C.S. Dickinson, J. Chem. Phys., 2004, 121, 9378]. Our best estimate of the potential curve results in vibrational energies with a RMS deviation of only ∼1 cm −1 for the entire set of all empirically determined vibrational levels known so far. These results represent a drastic improvement over previous theoretical predictions of vibrational levels of 7 LiH up to dissociation, D 0 , which was predicted to be 19594 cm −1 . † This work has been first presented at the 50th Sanibel Symposium, St. Simons Island, Georgia, USA, between February 24, and March 2, 2010. Electronic Supplementary Information (ESI) available: Potential energy curve data used to produce the final vibrational levels. See