Singularity band of velocity auto correlation function of Lennard-Jones fluid in complex $\omega$-plain

TL;DR

This paper investigates the complex frequency singularities of the velocity autocorrelation function in Lennard-Jones fluids, revealing a branch cut structure that persists across various temperatures, indicating a multi-valued nature of the correlation function.

Contribution

It demonstrates that the velocity autocorrelation function of Lennard-Jones fluids exhibits branch cuts in the complex plane, challenging the simple pole models and suggesting a complex multi-valued analytical structure.

Findings

Branch cuts form the singularity structure of Z(ω)

The gap between branch cuts and real axis is well-defined

Structure remains stable at high temperatures

Abstract

It is well known from the quantum theory of strongly correlated systems that poles (or more subtle singularities) of dynamic correlation functions in complex plane usually correspond to the collective or localized modes. Here we address singularities of velocity autocorrelation function $Z$ in complex $\omega$-plain for the one-component particle system with isotropic pair potential. We have found that naive few poles picture fails to describe analytical structure of $Z(\omega)$ of Lennard-Jones particle system in complex plain. Instead of few isolated poles we see the singularity manifold of $Z(\omega)$ forming branch cuts that suggests Lennard-Jones velocity autocorrelation function is a multiple-valued function of complex frequency. The brunch cuts are separated from the real axis by the well-defined "gap". The gap edges extend approximately parallel to the real frequency axis. The singularity structure is very stable under increase of the temperature; we have found its trace at temperatures even several orders of magnitude higher than the melting point. Our working hypothesis that the branch cut origin is related to the "interference" in $Z$ of one-particle kinetics and collective hydrodynamic motion.