Implicit water constraints silently break conjugate gradient mimization
There was some discussion on gmx-users of systems solvated in water that users
were trying to use CG energy mimization on. They ran into problems with grompp
double-checking input for internal consistency...
ERROR: can not do Conjugate Gradients with constraints (201522)"
which was thought to be caused by the implicit constraints in the rigid water
model, since the user had specified no constraints in the .mdp file. One user
reported that adding "-DFLEXIBLE" to their .mdp file allowed CG EM to run.
David Mobley and I thought that explicit discussion of this in the manual should
be considered -
1) in the CG section, a note that rigid water models won't work because of the
explicit constraints, and the need to overcome this with -DFLEXIBLE, if suitable
2) a mention of water constraints in the constraints section(s)
3) a more descriptive error message when using grompp to set up CG with EM if
the constraints are only associated with a rigid solvent
Any of the above will help a user recognise the problem.