A hybrid quantum mechanical force field molecular dynamics simulation of liquid methanol: Vibrational frequency shifts as a probe of the quantum mechanical/molecular mechanical coupling

Abstract

A hybrid quantum mechanical molecular dynamics method is used to study liquid methanol at room temperature and normal density. Frequencies of the twelve vibrational modes are calculated from the simulation data at the ab initio Hartree–Fock/3-21G(d,p) level. Good overall agreement is found between the experimental and calculated frequencies. Three different, successive levels of quantum mechanical/molecular mechanical (QM/MM) coupling schemes are investigated using gas-to-liquid vibrational frequency shifts as a probe. The results suggest, somewhat surprisingly, that the method with the weakest QM/MM coupling gives the best overall agreement between the experimental and simulated results for vibrational frequency shifts. The most elaborate coupling scheme overestimates the shifts towards the red direction due to overestimation of the attractive interactions between quantum mechanical and molecular mechanical molecules, while it is found to be most successful in describing the O–H stretch. The effects of the solvent on the geometrical parameters of methanol are investigated in detail.