Negative contributions in the velocity correlation function of supercooled liquid water

The translational dynamics of supercooled and normal liquid water is investigated via a specific correlation function DeltaB with the aim of explaining the behavior of the centers of mass velocity correlation function (VCF). DeltaB is divided into diffusive and nondiffusive parts that yield separated contributions to the VCF, namely an Enskog-type diffusive one, modeled by an exponential function, and a nondiffusive one, made up by damped oscillations of a vanishing time integral. In the translational density of states (DOS), the oscillations yield the bands at omega(1) congruent with 50 cm(-1), omega(3) congruent with 240 cm(-1) (the two well-known experimental bands of the Raman spectra) and omega(2) congruent with 160 cm(-1) (the Einstein frequency of the liquid). It is shown that the chief negative lobe of the VCF is mainly due to the DOS component at the lowest frequency omega(1). The study of the relative pair dynamics shows that this lobe is due to the transverse dynamics, while the longitudinal one determines the fast DOS component at omega(3). The presence of a negative tail is highlighted. Its contribution extends beyond the region of the fast dynamics (t<0.7 ps) up to about 1.5 ps and is due to a low-frequency oscillating mode that produces a low-frequency DOS band centered at about omega(0)=20 cm(-1).