Contribution of shears on vibrational entropy

TL;DR

This paper extends the calculation of vibrational entropy in defect studies by including shear effects in elastic corrections, showing significant impact on low-symmetry configurations.

Contribution

It introduces the inclusion of shears into elastic corrections for vibrational entropy, improving accuracy in defect formation and migration studies.

Findings

Shear inclusion significantly affects elastic corrections.

Impact is notable for low-symmetry defect configurations.

Method tested on copper defect models.

Abstract

Calculating the vibrational entropy of an N-atom assembly in the harmonic approximation requires the diagonalisation of a large matrix. This operation becomes rapidly time consuming when increasing the dimensions of the simulation cell. In the studies of point defects, a widely used shortcut consists in calculating the eigen modes of the atoms contained in an inner region, called the defective region, while the atoms belonging to the outer region are held fixed, and in applying an elastic correction to account for the entropy stored in the distortion of the outer region. A recent paper proposed to base the correction on the local pressure change experienced by each lattice site. The present contribution is an extension in the sense that it includes the shears. We compared the two approximations for configurations which are currently encountered in defect studies, namely those pertaining to defect formation and migration. The studied defects are the single, di- and tri-vacancy as well as the dumbbell interstitial in a host matrix modelled by several empirical potentials mimicking pure copper. It is shown that the inclusion of shears brings a noticeable contribution to the elastic correction for all configurations of low symmetry.