Calculation of Elastic constant of graphene

Hello everyone,

I have built a single layer of graphene sheet with 252 atoms using CVFF potential. I want to calculate elastic modulus and poison ratio for graphene for uniaxial tension in x direction. I used fix deform command to get the stress-strain curve. For example, If I use the following command

fix 9 all deform 1 x erate ${erate1} units box remap x

I get 0 strain in y-direction. So, poison ratio is also 0. Also, volume increases. But, if I use the following command,

fix 9 all deform 1 x erate ${erate1} y volume units box remap x

the volume remains conserved and also, I have some strain in y-direction, and the poison ratio at fracture strain is 0.77. But the graphene sheet itself have a wavy pattern throughout the simulation-not flat. Can you please help me understand the behavior of graphene here?