elastic constants

Hi lammps users,

I am not very clear about the deformation method in Aidan's script for calculating elastic constants. Why change the box shape in xy and xz directions when dir=1 and in yz direction when dir=2? Why does the changing distance equal deltaxy, deltaxz, or deltayz, not delta?

Thank you for your help in advance.


Take dir=1. This corresponds to the strain component \epsilon_{xx}, or axial strain in the x direction. The entire box and the atom positions are scaled uniformly in x by the factor (1+{up}). This requires changing xy by an amount deltaxy={up}*xy, etc. If this is not clear, it might help to recall that in LAMMPS, xy is the x-component of the second periodic cell vector, sometimes called b_x.