Relative stabilities of the two isomers of the methanol-water dimer: The effects of internal rotations of the hydroxyl and methyl groups of methanol

The ground-state energies of the two isomers of the methanol-water dimer, w ith water-donor (WM) and methanol-donor (MW) structures, have been calculat ed using the diffusion Monte Carlo (DMC) method with constraint dynamics. U nlike the rigid-body DMC, this method permits the, internal rotations of th e hydroxyl and methyl groups of methanol about the C-O bond. The DMC calcul ations-were performed for the isotopomers CH3OH . . .H2O, CH3OD . . .H2O, a nd CD3OH . . .H2O. The calculations with the internal rotation of the methy l and hydroxyl groups of methanol included resulted in a much larger ground -state energy gap between the WM and MW isomers than those in which these i nternal rotations were frozen. This result demonstrated the critical import ance of including the internal hydroxyl and methyl rotations in the DMC cal culations aimed at predicting accurately the relative stabilities of the tw o isomers of the methanol-water dimer. (C) 2001 American Institute of Physi cs.