  We introduce an improved semiclassical dynamics approach to quantum
vibrational spectroscopy. In this method, a harmonic-based phase space sampling
is preliminarily driven toward non-harmonic quantization by slowly switching on
the actual potential. The new coordinates and momenta serve as initial
conditions for the semiclassical dynamics calculation, leading to a substantial
decrease in the number of chaotic trajectories to deal with. Applications are
presented for model and molecular systems of increasing dimensionality
characterized by moderate or high chaoticity. They include a bidimensional
Henon-Heiles potential, water, formaldehyde, and methane.The method improves
accuracy and precision of semiclassical results and it can be easily interfaced
with all pre-existing semiclassical theories.
