Pressure-dependent structure of neat liquid methanol, CH3OH: Molecular dynamics simulations with various united atom-type potentials

Molecular dynamics computer simulations have been conducted on neat liquid methanol, using three different "united atom" (three site) interatomic potentials: TraPPE [Chen et al., J. Phys. Chem. B 105, 3093 (2001)], UAM-I [García-Melgarejo et al., J. Mol. Liq. 323, 114576 (2021)], and OPLS/2016 [D. Gonzalez-Salgado and C. Vega, J. Chem. Phys. 145, 034508 (2016)]. The effects of pressure, between 1 bar and 6 kbar, have been evaluated on total scattering structure factors, partial radial distribution functions, and on collective characteristics such as ring-size distributions and cluster-size distributions. Agreement with experimental density is nearly quantitative for all three force fields, and major trends observed for recent pressure-dependent neutron diffraction data are reproduced qualitatively. In general, the OPLS/2016 force field generates properties that are markedly different from results originating from the other potentials. Pressure effects are hardly noticeable on most partial radial distribution functions and on the distribution of the number of hydrogen-bonded neighbors. On the other hand, collective structural properties, such as cluster- and ring-size distributions, exhibit significant changes with increasing pressure: larger clusters become more numerous, whereas the number of cyclic clusters, i.e., rings, decreases. The self-diffusion coefficient decreases with increasing pressure, and the same is valid for the average lifetime of hydrogen bonds.