Testing the fourth moment approximation of vibrational mode lifetimes in fcc Lennard-Jonesium

Abstract

A recently proposed method of approximating the vibrational mode lifetimes in insulators is based on ensemble averages of displacements and forces. In this report, the so-called ‘fourth-moment approximation’ is tested against the conventional ‘ensemble molecular dynamics’ method on vibrational modes in fcc ‘Lennard-Jonesium’. Cells of 4, 8, 16, and 32 atoms are used as a testbed for comparing the two methods. The results show that the moments method gives a quite reasonable accounting of the vibrational mode lifetimes in these cells at all temperatures and is orders of magnitude faster than the MD-based approach.