Efficient Semiclassical Dynamics for Vibronic Spectroscopy beyond Harmonic, Condon, and Zero-Temperature Approximations

Understanding light-induced processes in biological and human-made molecular systems is one of the main goals of physical chemistry. It has been known for years that the photoinduced dynamics of atomic nuclei can be studied by looking at the vibrational substructure of electronic absorption and emission spectra. However, theoretical simulation is needed to understand how dynamics translates into the spectral features. Here, we review several recent developments in the computation of vibrationally resolved electronic spectra (sometimes simply called "vibronic" spectra). We present a theoretical approach for computing such spectra beyond the commonly used zero-temperature, Condon, and harmonic approximations. More specifically, we show how the on-the-fly ab initio thawed Gaussian approximation, which partially includes anharmonicity effects, can be combined with the thermo-field dynamics to treat non-zero temperature and with the Herzberg-Teller correction to include non-Condon effects. The combined method, which can treat all three effects, is applied to compute the S1 ← S0 and S2 ← S0 absorption spectra of azulene.

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

On-the-fly ab initio semiclassical evaluation of vibronic spectra at finite temperature

Tomislav Begusic, Jiri Vanicek

To compute and analyze vibrationally resolved electronic spectra at zero temperature, we have recently implemented the on-the-fly ab initio extended thawed Gaussian approximation [A. Patoz et al., J. Phys. Chem. Lett. 9, 2367 (2018)], which accounts for anharmonicity, mode–mode coupling, and Herzberg–Teller effects. Here, we generalize this method in order to evaluate spectra at non-zero temperature. In line with thermo-field dynamics, we transform the von Neumann evolution of the coherence component of the density matrix to the Schrödinger evolution of a wavefunction in an augmented space with twice as many degrees of freedom. Due to the efficiency of the extended thawed Gaussian approximation, this increase in the number of coordinates results in nearly no additional computational cost. More specifically, compared to the original, zero-temperature approach, the finite-temperature method requires no additional ab initio electronic structure calculations. At the same time, the new approach allows for a clear distinction among finite-temperature, anharmonicity, and Herzberg–Teller effects on spectra. We show, on a model Morse system, the advantages of the finite-temperature thawed Gaussian approximation over the commonly used global harmonic methods and apply it to evaluate the symmetry-forbidden absorption spectrum of benzene, where all of the aforementioned effects contribute. I. INTRODUCTI

2020

Efficient ab initio semiclassical dynamics for linear and nonlinear electronic spectroscopy

Tomislav Begusic

EPFL2021