Computation of the S1 ← S0 vibronic absorption spectrum of formaldehyde by variational Gaussian wavepacket and semiclassical IVR methods

The vibronic absorption spectrum of the electric dipole forbidden and vibronically allowed S1(1A2) ← S0(1A1) transition of formaldehyde is calculated by Gaussian wavepacket and semiclassical methods, along with numerically exact reference calculations, using the potential energy surface of Fu, Shepler and Bowman [J. Am. Chem. Soc. 133, 7957 (2011)]. Specifically, the variational Multiconfigurational Gaussian (vMCG) approach and the Herman-Kluk Semiclassical Initial Value Representation (HK-SCIVR) are compared to assess the accuracy and convergence of these methods, benchmarked against numerically exact wavepacket (TDWP) propagation results on the reference potential energy surface. The vMCG calculation is shown to converge quite well with about 100 variationally evolving Gaussian functions and using a local cubic expansion instead of the conventional local harmonic approximation. By contrast, the HK-SCIVR approach with ~105 trajectories reproduces the vibrationally structured spectral envelope correctly, ...