H-BOND IN METHANOL - A MOLECULAR-DYNAMICS STUDY

A set of molecular dynamics (MD) simulations of methanol-d4 at three temperatures in the liquid range (200, 250 and 300 K) have been carried out. The equations of motion of 256 molecules interacting through a potential model due to Haughney et al. [J. Phys. Chem., 91 (1987) 4934] were solved using the velocity version of the Verlet algorithm. This rather large number of molecules was required for studying the behaviour of the system at momentum transfers as low as 0.25 angstrom-1. It was found that the system experiences long period fluctuations, and therefore very long MD runs (of the order of 100 ps) are necessary in order to obtain accurate statistical averages. Computed static properties are in good agreement with those reported by Haughney et al. and the neutron weighted g (r) and the static structure factor compare favourably with available neutron diffraction data. The study of time-dependent properties through centre-of-mass autocorrelation functions (VACF, F(s)(Q,t) and F(Q,t)) and their memory functions reveals features unknown in simple liquids and very similar to those found in liquid water. A close agreement between centre-of-mass single-particle autocorrelation functions and the translational part of QENS data is also observed. The dynamic structure factor for the centres of mass show distinctive side peaks in the same region of the (Q, omega) plane where recently collective excitations have been studied using coherent neutron scattering thus establishing the presence of propagating short wavelength modes.