It was noted that calculating the vibrational entropy of an N-atom assembly in the harmonic approximation required the diagonalization of a large matrix. This operation rapidly became time-consuming upon increasing the dimensions of the simulation cell. In studies of point defects, a widely-used short-cut consisted of calculating the eigenmodes of the atoms contained in an inner region (the defect region) while the atoms belonging to the outer region were held fixed, and in applying an elastic correction in order to account for the entropy stored in the distortion of the outer region. It had been proposed that the correction should be based upon the local pressure-change experienced by each lattice site. The present method was an extension of this, in that it included shears. The 2 approximations were compared for configurations which were commonly encountered in defect studies: defect formation and migration. The studied defects were the single, di- and trivacancy, as well as the dumb-bell interstitial, in a host matrix modelled by several empirical potentials modelling pure Cu. The inclusion of shears added a noticeable contribution to the elastic correction of all low-symmetry configurations.

Contribution of Shear Strains to the Vibrational Entropy of Defect Configurations. J.L.Bocquet: Philosophical Magazine, 2007, 87[22], 3259-95