Ab initio molecular dynamics study of collective excitations in liquid H2O and D2O: Effect of dispersion corrections

Abstract

The collective dynamics in liquid water is an active research topic experimentally, theoretically and via simula- tions. Here, ab initio molecular dynamics simulations are reported in heavy and ordinary water at temperature 323.15 K, or 50°C. The simulations in heavy water were performed both with and without dispersion correc- tions. We found that the dispersion correction (DFT-D3) changes the relaxation of density-density time corre- lation functions from a slow, typical of a supercooled state, to exponential decay behaviour of regular liquids. This implies an essential reduction of the melting point of ice in simulations with DFT-D3. Analysis of longitudi- nal (L) and transverse (T) current spectral functions allowed us to estimate the dispersions of acoustic and optic collective excitations and to observe the L-T mixing effect. The dispersion correction shifts the L and T optic (0) modes to lower frequencies and provides by almost thirty per cent smaller gap between the longest-wavelength LO and TO excitations, which can be a consequence of a larger effective high-frequency dielectric permittivity in simulations with dispersion corrections. Simulation in ordinary water with the dispersion correction results in frequencies of optic excitations higher than in D2O, and in a long-wavelength LO-TO gap of 24 ps-l (127 cm-1).