Why dosn't the cell length change under pressure?

Hi all,

I simulated a periodic box including a graphene sheet at the middle (Fig. is attached). I did some NPT dynamics under the atmosphere pressure, but the length of the box didn't reduced in C direction (normal to graphene). This also happens when some polymer chains are packed into the empty space of the box with low density (e.g. 0.5 g/cm3). What is the reason?


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Thank you in advance.
Farzin