Ab initio molecular dynamics study of collective excitations in liquid H$_2$O and D$_2$O: Effect of dispersion corrections

TL;DR

This study uses ab initio molecular dynamics to analyze how dispersion corrections affect collective excitations and dynamics in liquid water and heavy water at 50°C, revealing significant changes in relaxation behavior and excitation spectra.

Contribution

It demonstrates that including dispersion corrections in ab initio simulations significantly alters the collective dynamics and excitation spectra of liquid water and heavy water.

Findings

Dispersion correction changes density correlation decay from slow to exponential.

Dispersion correction shifts optical mode frequencies to lower values.

Long-wavelength LO-TO gap is approximately 24 ps$^{-1}$ in ordinary water.

Abstract

The collective dynamics in liquid water is an active research topic experimentally, theoretically and via simulations. Here, ab initio molecular dynamics simulations are reported in heavy and ordinary water at temperature 323.15 K, or 50$^\circ$C. The simulations in heavy water were performed both with and without dispersion corrections. We found that the dispersion correction (DFT-D3) changes the relaxation of density-density time correlation 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 longitudinal (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 (O) 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 D$_2$O, and in a long-wavelength LO-TO gap of 24 ps$^{-1}$ (127 cm$^{-1}$).