[gmx-users] how to fix the center of mass of the three N-terminal atoms in a triple helix?

[Ming Tang]

Deer all,
I am trying to use the steered molecular dynamic method to study a triple helix.
According to a paper, it can be done through fixing the COM of the three terminal by means of a strong harmonic
restrain with a super large spring constant, and pull the C-terminal by an appropriate spring constant in Gromos96 53a6.
But I still do not know how to keep the COM fixed.
Could anybody give me same guidance?

Thanks a lot,
Ming