I started to simulate graphene and try to equilibrate the sheet with fix npt command. But I found that during the barostat the sheet oscillated a lot and it looks like a ripple on the graphene.
The boundary condition is p p s. I used airebo potential. The fix npt command is used to control the pressure at the periodic boundaries to be zero. And some key commands I used are listed as the following. Please find the input file in the attachments.
pair\_style airebo 3\.0 pair\_coeff \* \* CH\.airebo C neighbor 2\.0 nsq min\_style cg minimize 0 0 100000 100000 fix npt\_relax all npt temp 300 300 0\.05 x 0 0 0\.5 y 0 0 0\.5 drag 0\.2
I can understand that at finite temperature the atoms moves around their lattice positions. But the max magnitude of oscillation along the z axis is about 5.5 angstrom in my simulation and it seems a bit high. I also changed the temperature of my simulation, at 10 Kelvin the magnitude is lower with about 2 angstrom. I feel weird that the magnitude also related to the size of the graphene. If I use a bigger sheet which contains about 4 times the number of atoms with total 80000 atoms, the max magnitude increased to about 9.5 angtrom at 300 Kelvin and 4 angtrom at 10 K. Please find the pictures of the oscillation in the attachments.
I checked the docs and tried to change some values in the commands, but the results are similar. The result is OK if I use these command to equilibrate bulk materials. I am not sure whether the oscillation of the graphene sheet is reasonable under the barostat? Do I misuse some commands or parameters? This looks like a very basic question but I am kind of getting stuck with this for weeks. Please guide me if you have any suggestions.
Many thanks in advance!
With My Best Regards,
in.equil (553 Bytes)