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 simulations. 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 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 ).

Pressure-induced effects in the spectra of collective excitations in pure liquid metals

“…It has been suggested that the emergence of transverse modes comes from contributions of the longitudinal ones [15,16], following a relationship between the L-and T-current spectral functions based on mode-coupling approach. Such an emergence of contributions from longitudinal excitations in the transverse spectral functions was revealed and documented in simulations of water [17,18]. For liquid metals, it has been shown [12][13][14] that the high frequency transverse modes coincide with the simultaneous emergence of a high frequency peak in the vibrational density of states.…”

Ab initio molecular dynamics study of collective dynamics in liquid Tl: Thermo-viscoelastic analysis

“…The calculated static structure factor S (k) has the same location of the first sharp diffraction peak, however with larger amplitude, than the experimental static structure factor for liquid Tl [21]. This fact can be a consequence of overbinding in local structure due to the applied PBE exchange-correlation, similar as this is usually observed in AIMD simulations of water [22].…”

Vibrational spectroscopies in liquid water: on temperature and coordination effects in Raman and infrared spectroscopies

“…From earlier studies it has become apparent that the BLYP+D3 approach yields relatively realistic description of liquid water. Yet short-comings persist, like the too high equilibrium density at ambient conditions [6], the modest over-structuring of the liquid [13] arising due to a too high melting temperature [31] evidenced also in the collective vibrations [32].…”