  We report on a new approach to ease the computational overhead of ab initio
on-the-fly semiclassical dynamics simulations for vibrational spectroscopy. The
well known bottleneck of such computations lies in the necessity to estimate
the Hessian matrix for propagating the semiclassical pre-exponential factor at
each step along the dynamics. The procedure proposed here is based on the
creation of a dynamical database of Hessians and associated molecular
geometries able to speed up calculations while preserving the accuracy of
results at a satisfactory level. This new approach can be interfaced to both
analytical potential energy surfaces and on-the-fly dynamics, allowing one to
study even large systems previously not achievable. We present results obtained
for semiclassical vibrational power spectra of methane, glycine, and
N-acetyl-L-phenylalaninyl-L-methionine-amide, a molecule of biological interest
made of 46 atoms.
