Semiclassical on-the-fly computation of the S(0)-->S(1) absorption spectrum of formaldehyde

The anharmonic S(0)-->S(1) vibronic absorption spectrum of the formaldehyde molecule is computed on the fly using semiclassical dynamics. This first example of an on-the-fly semiclassical computation of a vibronic spectrum was achieved using a unit prefactor modified frozen Gaussian semiclassical propagator for the excited state. A sample of 6000 trajectories sufficed for obtaining a converged spectrum, which is in reasonable agreement with experiment. Similar agreement is not obtained when using a harmonic approximation for the spectrum, demonstrating the need for a full anharmonic computation. This first example provides a resolution of approximately 100 cm(-1). Potential ways of improving the methodology and obtaining higher resolution and accuracy are discussed.