[gmx-users] absolute reference for pulling and center of mass removal

[Bert]

Dear gmx-users，

I intend to pull a polymer in the bulk water using the pull code in
gmx4. When I checked the manual, it was said " With an absolute
reference the system is no longer translation invariant and one should
think about what to do with the center of mass motion." Yes, I set the
pull group with the polymer and the reference group with the default
(0,0,0). Then I received a warning after grompp as follows:
"You are using an absolute reference for pulling, but the rest of the
system does not have an absolute reference. This will lead to
artifacts."

I am not sure about the artifacts artifacts, could somebody give me a
more definite explanation.

Assuming we pull the COM of the polymer along x, and then the COM of
water in x is coupled with that of the polymer, but the COM of water
in yz is uncoupled. As a result, the momentum of system in yz could be
nonzero for long time simulations if no COM removal is applied in yz.
So to speak, if we do not remove the COM motion, artifacets could be
induced by round errors in uncoupled yz direction. Am I right?

How the deal with the COM removal in this case?

I have done two tests here:

1) Set comm-grps = system. The polymer and the water flowed in the
negative direction.
2) Set comm-grps = polymer SOL. The polymer still moved in x, and no
COM motion observed for SOL.

Which one could be appropriate, and if both are not ok, is there a way
to solve the problems? Thanks for your suggestions in advance!

Best regards,
Bert