The structure of liquid methanol: a molecular dynamics study using a six-site model

Molecular dynamics (MD) simulations of pure methanol (216 molecules) have been carried out at 298.15 K in the NVE ensemble using a six-site potential model originally derived by Anwander et al (1992 Chem. Phys. 166 341) from ab initio quantum chemical calculations (QCC) and tested for the first time in this study. MD results of a three-site model where all the methyl hydrogens were considered as a dead load have also been reported recently by us. In this paper, the relative merits of the two models are discussed by comparing the simulated radial distribution functions (rdfs) with the recent experimental neutron diffraction (ND) results obtained at the partial pair distribution function (pdf) level. Although the MD simulations with both the models reproduce the total rdfs rather well, discrepancies begin to appear at the partial pdf level. Both the simulations are found to reproduce equally well the X-X (X = C, O or H, a methyl hydrogen) pdf since it comprises six correlations, and is dominated mainly by contributions from the methyl group. However, the main peaks of the simulated H-O-H-O partial,where H-O is the hydroxyl hydrogen, are found to be slightly higher and shifted to larger distances as compared to the ND results. A comparison of the simulated X-H-O intermolecular rdf, in which H-H-O correlations dominate, with the ND results shows that, although the three-site model reproduces at least qualitatively the experimental features, the six-site model derived from ab initio QCC fails badly.