Temperature evolution of the translational density of states of liquid water

Abstract

The influence of temperature and density functional models in ab initio molecular dynamics simulation of liquid waterThe molecular dynamics technique is used to study the relative dynamics of tagged pairs of molecules and to derive the related translational density of states ͑DOS͒ of liquid water at 243, 273, and 373 K. The modes that compose the short-time dynamics of centers of mass are obtained. The dynamical quantities studied are characterized by a fast-time decay followed by a plateau whose height increases with the temperature and with the initial pair separation. The plateau is attributed to the nonharmonic motions and its height is related to the pair relative diffusion coefficient. An exponential relaxation is used to represent the way the system follows to reach the diffusive behavior; the derived relaxation times agree with those reported in the literature describing the fast translational dynamics. The frequencies of the other short-time modes are related to the main frequencies of the solid, while the mode damping is analyzed in terms of the damped harmonic oscillator model; it is found that the Gaussian damping gives a better fit to the DOS than the exponential one. The temperature evolution of the various modes is investigated and related to the microscopic pair dynamics. In particular, the modes at low frequencies ͑Х50 cm Ϫ1 ͒ are absent in the vibrations along the hydrogen bond ͑longitudinal modes͒; they are present in the transverselike dynamics of other pair states. The temperature increase produces the intensity decrease of the 50-cm Ϫ1 band and the pile-up of the DOS intensity towards zero frequency. The decay constants of these two effects have a different temperature dependence, which confirms the oxygen bending nature of the 50-cm Ϫ1 band and its independence on the relaxational-like dynamics.

Comment on “An interpretation of the low-frequency spectrum of liquid water” [J. Chem. Phys. 118, 452 (2003)]

“…The same bands compose the DOS of liquid water. 12 The comparison argon-water over the reduced frequency scale shows that the main DOS difference is due to the intensities of the two bands. The longitudinal dynamics of neighbor particles of the two fluids is similar: the hindrance of the cage in a LJ system corresponds to the HB action in water, the cage being formed by the four H-bonded first neighbors.…”

“…These results can be extended to the supercooled and melting liquids of argon and water, since the same three bands compose the total DOS of the two liquids. 11,12 The association of the 60 cm Ϫ1 band with the transverse dynamics was performed via a density current approach 14 16 or that the structural defects of the HB network, e.g., the bifurcated H bonds, 15 give spectral contributions near 60 cm Ϫ1 . Beyond these details, one can conclude that ͑a͒ the two bands that in water fall at about 60 cm Ϫ1 and 240 cm Ϫ1 are also present in argon liquid at the melting and supercooled states.…”

Negative contributions in the velocity correlation function of supercooled liquid water

“…A detailed analysis of the static and dynamical properties of water via g(r,⍀ 1 ,⍀ 2 ) was performed elsewhere. 17,18 To obtain the time correlation functions ⌬B 19 and the VCF, molecular dynamics simulations at NVE conditions were carried out for 864 molecules interacting via the TIP4P potential 20 in a cubic box with periodic boundary conditions. The configurations of the liquid at the triple point (Tϭ273 K, ϭ1.0 g/cm 3 ) ͑melting state for simplicity͒ and of the supercooled state (Tϭ245 K, ϭ1.0 g/cm 3 ) were produced.…”

Excess of low-frequency modes in Lennard-Jones systems

“…The evidence of a DOS excess is obtained by comparing the behaviour of disordered, crystalline and Debye-like systems, while the boson peak connection is suggested by the time decrease of the low-frequency mode intensity and by its disappearance when the crystallization process is over. These results are obtained by applying a methodology proposed for supercooled water states [20] and recently extended to normal liquid states [21].…”

“…The term DOS is used to indicate the power spectrum of ω B(r 0 ; t) that yields part of the total DOS defined as the cosine Fourier transform of the velocity autocorrelation function[21] 5. It can be shown that this is true in the cases of water[21] and argon[22] liquid.…”