boundary conditions

Dear All,

I am doing a tension test on graphene honeycomb at 0 K with pps boundary conditions. Few layers of atoms at the ends of the sample in z-direction have frozen degrees of freedom. Tensile displacement is applied to the frozen layers in increments with energy minimization after each increment.
To allow Poisson’s contraction in x- and y-directions, I specify zero pressure along those directions. However, the frozen layers separate from the sample after few displacement increments along z-direction. If no pressure is specified along x- and y-direction, then the frozen layers do not separate, and the sample deform without Poison’s contraction.

Does anybody have an idea of whats going in the sample with zero-pressure specified in x- and y-directions. I am attaching the input, potential and data files with this email.

sivashankar (228 KB)