Gas phase infrared spectra from quasi-classical Kubo time correlation functions

We generalise the recently developed phase integration method (PIM) to obtain a computable approximation of the Kubo expression for quantum time correlation functions. Our scheme combines exact sampling of the quantum thermal density with classical dynamics to provide a quasi-classical approximation for the correlation function. The method will be specialised to the evaluation of the momentum autocorrelation function, with the goal to compute infrared spectra of simple molecules in the gas phase. Application to two simple but interesting benchmark systems shows that the approach is accurate and stable over a broad range of temperatures.