Title: Ab initio molecular dynamics study of collective excitations in liquid H₂O and D₂O: Effect of dispersion corrections
Author(s):

	T. Bryk (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii St., 79011 Lviv, Ukraine; Institute of Applied Mathematics and Fundamental Sciences, Lviv Polytechnic National University, 79013 Lviv, Ukraine) ,
	A.P. Seitsonen (Institut fur Chemie, Universitat Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland; Departement de Chimie, Ecole Normale Superieure, 24 rue Lhomond, F-75005 Paris, France)

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^oC. 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 D₂O, and in a long-wavelength LO-TO gap of 24 ps^-1 (127 cm^-1).

Key words: collective excitations, optic modes, water, heavy water, van der Waals corrections, ab initio molecular dynamics
PACS: 61.20.Ja, 61.20.Lc, 62.60.+v, 63.50.-x, 78.30.C-

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