The hidden microscopic dynamics of liquid methanol unveiled by a combined analysis of MD simulations and inelastic neutron scattering data

Methanol has long deserved, like water, the interest of many scientists, both from the chemical and the physical point of view. Innumerable papers deal with the peculiar properties of methanol which, as one of the emblems of hydrogen-bonded liquids, behaves rather differently from simple molecular liquids. In addition, the studies of its mixtures with water still leave many open questions, thus explaining the unfaded interest in these systems. As regards the dynamics of pure methanol, the single-molecule behaviour was investigated with some success by several neutron scattering (NS) measurements, while the same technique gave less accurate information about the collective excitations. Rather aged simulation results for S(Q,?) indeed appeared to reveal quite weak inelastic modes in methanol, difficult to disentangle from the overwhelming quasi-elastic signal in the spectra, already at Q values as low as 5 nm^-1. Two modes were roughly observed, but the computing limitations at that time, combined with neutron data of difficult analysis, left the collective dynamics of methanol substantially unknown and not well interpreted. Here we present novel simulation and NS data on deuterated methanol at room temperature, in a Q range extending from 2 to 20 nm^-1 (i.e. above the main peak in the static structure factor). The collective excitations of this liquid are clearly disclosed, and shown to follow, with very similar features, the infinitely discussed "two-mode" dynamics observed in water. The self dynamics is also revisited here, with some novelty.