On-the-fly ab initio semiclassical evaluation of time-resolved electronic spectra

We present a methodology for computing vibrationally and time-resolved pump-probe spectra, which takes into account all vibrational degrees of freedom and is based on the combination of the thawed Gaussian approximation with on-the-fly ab initio evaluation of the electronic structure. The method is applied to the phenyl radical and compared with two more approximate approaches based on the global harmonic approximation the global harmonic method expands both the ground- and excited-state potential energy surfaces to the second order about the corresponding minima, while the combined global harmonic/on-the-fly method retains the on-the-fly scheme for the excited-state wavepacket propagation. We also compare the spectra by considering their means and widths, and show analytically how these measures are related to the properties of the semiclassical wavepacket. We find that the combined approach is better than the global harmonic one in describing the vibrational structure, while the global harmonic approximation estimates better the overall means and widths of the spectra due to a partial cancellation of errors. Although the full-dimensional on-the-fly ab initio result seems to reflect the dynamics of only one mode, we show, by performing exact quantum calculations, that this simple structure cannot be recovered using a one-dimensional model. Yet, the agreement between the quantum and semiclassical spectra in this simple, but anharmonic model lends additional support for the full-dimensional ab initio thawed Gaussian calculation of the phenyl radical spectra. We conclude that the thawed Gaussian approximation provides a viable alternative to the expensive or unfeasible exact quantum calculations in cases, where low-dimensional models are not sufficiently accurate to represent the full system. (C) 2018 Author(s).

Efficient ab initio semiclassical dynamics for linear and nonlinear electronic spectroscopy

Tomislav Begusic

Molecular spectroscopy is an essential experimental technique in both fundamental physical sciences and applied research. For example, electronic spectroscopy, in which light induces a change in the electronic state of a molecule, is a powerful tool for studying light-matter interactions appearing in solar energy conversion and light-emitting devices. To explain experimental findings and compare them with theoretical predictions, scientists often compute properties that can only be inferred indirectly, rather than simulating the observed spectroscopic signals. This is partly due to challenges associated with the simulation of electronic spectra, which requires both an accurate description of the electronic structure and the quantum-dynamical treatment of the motion of atomic nuclei. As the exact methods to solve the underlying quantum-mechanical problem scale exponentially with the number of atoms, this task must be performed approximately for all but the smallest molecules. In this thesis, we develop a set of methods to systematically study the effects of different approximations on the computed spectra. As our starting point, we consider a semiclassical method called the thawed Gaussian approximation, which has resurfaced in recent years as an efficient and robust alternative to simple but crude models or highly accurate but costly quantum dynamics methods. First, we present several practical improvements of the method, such as generalization to non-Condon effects or the efficient single-Hessian version, which enable a broader set of molecules to be studied. Second, we introduce a more fundamental modification of the methods - the inclusion of non-zero temperature effects. The underlying theory, which is inspired by the concept of so-called thermo-field dynamics, is exact and applies to any quantum dynamics method. Remarkably, within the thawed Gaussian approximation, the inclusion of non-zero temperature comes at nearly no additional computational cost. These methods, developed originally for steady-state linear spectra, are then employed to compute state-of-the-art, time-resolved spectra, which are typically measured with ultrashort laser pulses. More precisely, we study how the anharmonicity of the potential energy surfaces and mode-mode (Duschinsky) couplings affect pump-probe and two-dimensional electronic spectra. This approach, based on the thawed Gaussian wavepacket propagation, is the first method that can exactly evaluate the nonlinear electronic spectra of many-dimensional, shifted, distorted, and Duschinsky-rotated harmonic potentials. As in the case of linear absorption and emission spectra, we again use the concept of thermo-field dynamics to include non-zero temperature effects in the simulation of two-dimensional electronic spectra. The resulting wavepacket picture of two-dimensional spectroscopy at arbitrary temperature could strongly impact how we interpret and simulate multidimensional spectra in the future.

EPFL2021

Ab initio semiclassical evaluation of vibrationally resolved electronic spectra with thawed Gaussians

Tomislav Begusic, Jiri Vanicek

Vibrationally resolved electronic spectra of polyatomic molecules provide valuable information about the quantum properties of both electrons and nuclei. This chapter reviews the recent progress in ab initio semiclassical calculations of such spectra, based on the thawed Gaussian approximation and its extensions. After reviewing molecular quantum dynamics induced by the interaction with electromagnetic field and the most common semiclassical approximations to quantum dynamics, we explain details of the thawed Gaussian approximation and its variants. Next, we discuss the time-dependent approach to steady-state and time-resolved electronic spectroscopy, and review several standard models that facilitate interpreting vibrationally resolved electronic spectra. Finally, we present the on-the-fly ab initio implementation of the thawed Gaussian approximation and provide several examples of both linear and pump-probe spectra computed with this methodology, which, at a low additional cost and without sacrificing the ease of interpretation, outperforms the standard global harmonic approaches.

Elsevier2021

Efficient evaluation of accuracy of molecular quantum dynamics using dephasing representation

Cesare Mollica, Jiri Vanicek

Ab initio methods for the electronic structure of molecules have reached a satisfactory accuracy for calculations of static properties but remain too expensive for quantum dynamics 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 the dephasing representation of quantum fidelity and its feasibility is demonstrated on the photodissociation dynamics of CO2. Our accuracy test can be easily implemented in existing molecular dynamics codes, thus offering wide applicability.

American Institute of Physics2009

Efficient ab initio semiclassical dynamics for linear and nonlinear electronic spectroscopy

Tomislav Begusic

EPFL2021