  We describe a new approach based on semiclassical molecular dynamics that
allows to simulate infrared absorption or emission spectra of molecular systems
with inclusion of anharmonic intensities. This is achieved from semiclassical
power spectra by computing first the vibrational eigenfunctions as a linear
combination of harmonic states, and then the oscillator strengths associated to
the vibrational transitions. We test the approach against a 1D Morse potential
and apply it to the water molecule with results in excellent agreement with
discrete variable representation quantum benchmarks. The method does not
require any grid calculations and it is directly extendable to high dimensional
systems. The usual exponential scaling of the basis set size with the
dimensionality of the system can be avoided by means of an appropriate
truncation scheme. Furthermore, the approach has the advantage to provide IR
spectra beyond the harmonic approximation without losing the possibility of an
intuitive assignment of absorption peaks in terms of normal modes of vibration.
