A HYBRID QUANTUM-MECHANICAL FORCE-FIELD MOLECULAR-DYNAMICS SIMULATIONOF LIQUID METHANOL - VIBRATIONAL FREQUENCY-SHIFTS AS A PROBE OF THE QUANTUM-MECHANICAL MOLECULAR MECHANICAL COUPLING

A hybrid quantum mechanical molecular dynamics method is used to study liquid methanol at room temperature and normal density. Frequencies o f the twelve vibrational modes are calculated from the simulation data at the ab initio Hartree-Fock/3-21G(d,p) level. Good overall agreemen t is found between the experimental and calculated frequencies. Three different, successive levels of quantum mechanical/molecular mechanica l (QM/MM) coupling schemes are investigated using gas-to-liquid vibrat ional frequency shifts as a probe. The results suggest, somewhat surpr isingly, that the method with the weakest QM/MM coupling gives the bes t overall agreement between the experimental and simulated results for vibrational frequency shifts. The most elaborate coupling scheme over estimates the shifts towards the red direction due to overestimation o f the attractive interactions between quantum mechanical and molecular mechanical molecules, while it is found to be most successful in desc ribing the O-H stretch. The effects of the solvent on the geometrical parameters of methanol are investigated in detail. (C) 1996 American I nstitute of Physics.