Umbrella sampling simulation

GROMACS version:2021.1
GROMACS modification: No
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Hi everyone,
I am trying to setup a umbrella simulation for a protein complex with two chains. When generating configurations as guided in Umbrella Sampling (, an error is displayed. Chain B is pulled along z-axis.
The error message is as follows:
Error 1 (dile MDP/md_pull.mdp)
When the maximum distance from a pull group reference atom to other atos in the group is larger than 0.5 times half the box size a centrally placed atom must be chosen as pbcatom. Pull group 2 is larger than that and does not have a specific atom selected as reference atom.

Your inputs on the error is greatly appreciated.

Thanks and Regards,

Warning is apparent, as maximum possible distance in a periodic system is half of box length. Either use larger box or smaller pull distance. Another way is to use pull-coord1-geometry=direction-periodic, which prohibits using Pressure coupler due to dynamic box.