V-T theory for the Self-Intermediate Scattering Function in a Monatomic Liquid

TL;DR

This paper applies V-T theory to accurately model the self-intermediate scattering function in a monatomic liquid, integrating vibrational and transit motions, and compares it with mode-coupling theory, showing close correspondence in atomic motion descriptions.

Contribution

The study extends V-T theory to the SISF with improved MSD calibration, providing a detailed three-interval model and demonstrating its accuracy across all wave numbers and times.

Findings

V-T theory accurately models SISF at all q and t.

Three-interval model effectively describes vibrational, crossover, and diffusive regimes.

V-T and mode-coupling theories show logical correspondence in atomic motions.

Abstract

In V-T theory the atomic motion is harmonic vibrations in a liquid-specific potential energy valley, plus transits, which move the system rapidly among the multitude of such valleys. In its first application to the self intermediate scattering function (SISF), V-T theory produced an accurate account of molecular dynamics (MD) data at all wave numbers q and time t. Recently, analysis of the mean square displacement (MSD) resolved a crossover behavior that was not observed in the SISF study. Our purpose here is to apply the more accurate MSD calibration to the SISF, and assess the results. We derive and discuss the theoretical equations for vibrational and transit contributions to the SISF. The time evolution is divided into three successive intervals: the vibrational interval when the vibrational contribution alone accurately accounts for the MD data; the crossover when the vibrational contribution saturates and the transit contribution becomes resolved; and the diffusive interval when the transit contribution alone accurately accounts for the MD data. The resulting theoretical error is extremely small at all q and t. Comparison of V-T and mode-coupling theories for the MSD and SISF reveals that, while their formulations differ substantially, their underlying atomic motions are in logical correspondence.