Abstract

To evaluate vibrationally resolved electronic spectra of polyatomic molecules, most modern electronic structure packages still employ the crude global harmonic approximation, and possibly add an ad hoc anharmonicity correction at the end. I will explain how such calculations can be improved simply but rigorously by combining Heller’s thawed Gaussian approximation (TGA) [1] with an on-the-fly ab initio (OTF-AI) scheme. This semiclassical methodology will be explained in general as well as on examples of absorption, emission, photoelectron, and resonance Raman spectra [2, 3, 4, 5]. On one hand, the efficiency of the OTF -AI-TGA permits treating all vibrational degrees of freedom explicitly and on an equal footing. (Up to 105-dimensional examples will be shown [2].) On the other hand, its improved accuracy succeeds in describing electronic absorption and photoelectron spectra of ammonia [3], chosen as a prototypical example of a floppy molecule exhibiting large amplitude motion, for which global harmonic approaches fail. I will explain how the method can be extended [4] in order to go beyond the Franck-Condon approximation and capture the coordinate dependence of the transition dipole; this will be demonstrated on the Herzberg-Teller absorption spectra of the phenyl radical and of benzene, a paradigm of an electronically forbidden, but vibronically allowed transition. Finally, I will show how a single semiclassical trajectory can be used to quantify the dynamical coupling between vibrational degrees of freedom and help decipher the spectral line shapes, even without using symmetry [2].

[1] E. J. Heller, J. Chem. Phys. 62, 1544 (1975). [2] M. Wehrle, M. Šulc, and J. Vaníček, J. Chem. Phys. 140, 244114 (2014). [3] M. Wehrle, S. Oberli, and J. Vaníček, J. Phys. Chem. A 119 , 5685 (2015). [4] A. Patoz, T. Begušić, and J. Vaníček, in preparation. [5] S. Reynaud, S. V. Antipov, and J. Vaníček, in preparation.