Rotational Memory Function of SPC/E water

TL;DR

This paper calculates the memory functions for dipole rotations in SPC/E water, showing that single-particle and collective dynamics are nearly identical, which supports existing dielectric spectroscopy theories.

Contribution

First calculation of memory functions for dipole rotations in SPC/E water, linking single-particle and collective relaxation times and validating dielectric spectroscopy models.

Findings

Memory functions for single and collective dipole dynamics are nearly identical.

Short memory time (~1 fs) supports rotational diffusion models.

Dielectric function contains no new dynamic information beyond single-dipole correlations.

Abstract

Memory effects are essential for dynamics of condensed materials and are responsible for non-exponential relaxation of correlation functions of dynamic variables through the memory function. Memory functions of dipole rotations for polar liquids have never been calculated. We present here calculations of memory functions for single-dipole rotations and for the overall dipole moment of the sample for SPC/E water. The memory functions for single-particle and collective dipole dynamics turn out to be nearly identical. This result validates theories of dielectric spectroscopy in terms of single-particle time correlation functions and the connection between the collective and single-particle relaxation times through the Kirkwood factor. The dielectric function in this formalism contains no new dynamic information that does not exist in the single-dipole correlation function. A short memory time, $\lesssim 1$ fs, justifies the use of rotational diffusion model to describe dynamics of a single molecular dipole moment in bulk water.