QUANTUM MONTE CARLO SIMULATION OF VIBRATIONAL FREQUENCY SHIFTS OF CO IN SOLID para-HYDROGEN

Stimulated by Fajardo’s remarkable study of the rovibrational spectra of CO isotopologues trapped in solid parahydrogen,a we have performed quantum Monte Carlo simulations to predict his observed vibrational frequency shifts and inertial rotational constants using 2-body potentials based on the best available models for the pH2-pH2 b and COpH2 c potential energy functions. We started by fitting an analytic ‘Morse/Long-Range’ (MLR) functiond to the 1D “adiabaic hindered rotor” version of Hinde’s 5D pH2-pH2 potential developed by Faruk et al.e We then modified it to take account of many-body effects by scaling it until it yielded the correct equilibrium lattice parameters for the fcc and hcp structures of pure solid para-hydrogen. A CO molecule was then placed at different interstitial or substitution sites in large equilibrated fcc or hcp para-hydrogen lattices, and the structural and dynamical behaviors of the micro-solvation environment around CO were simulated with a PIMC algorithm using a 2D effective pH2-CO potential based on the 5D H2–CO potential energy surface recently reported by Li et al.,f with a lattice sum of values of the 2D CO vibrational difference potential being use to predict the vibrational frequency shift. The effective rotational constants Beff for CO in different solid para-hydrogen structures were also calculated and compared with the experimental observations and with predicted Beff values for CO in large-sized para-hydrogen–CO clusters.g