Rotational dynamics of CO solvated in small He clusters: A quantum Monte Carlo study

Abstract

Abstract The reptation QMC method introduced by Baroni and Moroni0 a few years earlier is demonstrated in this paper to be the method of choice for predicting the rotational-vibrational features of the infrared spectrum of CO in small helium droplets. The calculations gave good agreement with experimental measurements for these relatively complex systems and provided insights into some of their characteristics not observable in experiments. The calculations were carried out for clusters containing one CO molecule and up to 30 He atoms. The CO molecule was treated as rigid, and HeHe and He-CO interactions were treated as pairwise additive, with pair terms from ab initio calculations. The trial wavefunction for this nodeless bosonic system was a Jastrow function made up of exponential functions dependent on the CO distance and orientation with respect to He atom and He-He distances. The reptation QMC calculations were carried out as diffusion QMC calculations with importance-sampling drift, but without multiplication so that the energy obtained was the expectation value of the energy for the trial wavefunction. The reptation part of the calculation produced exact (or nearly exact) expectation values and, more important in these calculations, unbiased quantum correlation functions from which the absorption spectra of the CO molecule could be determined. The results exhibit a variety of interesting phenomena. The binding energy for an additional atom first increases and then decreases before leveling out with increasing cluster size. The calculated rotational absorption spectra show two types of lines, one increasing in strength with cluster size, the other decreasing with cluster size. Much more detail, especially in regard to the droplet shape, revealed in the calculations.