[gmx-users] call for help

[曹臻]

Hello, everyone,

I meet a problem when I perform the gromacs. 
I fixed two C60 molecule at a certain distance(2.02 nm) by SHAKE algorithm. The
atoms have no charge on them. So the interaction between the molecules should be
the van der Waals interactions. I set the van der Waals cut off to be 1.2nm. The
intramolecular interaction was set to be zero. The intermolecular distance should
be further than 1.2nm. The length of the pbc box was 4.7nm. I run pullcode within
NVE ensemble. The ensemble average of force between the two C60 molecules should
be zero.  However, it is not. What mistake did I commit?

Thank you in advance.

Cao, Zhen