Force constant for distance restraints


I am not clear about the force constant for distance restraints under section [distance_restraints],

; ai aj type index type’ low up1 up2 fac

About fac, according to the manual: “In some cases it can be useful to have different
force constants for some restraints; this is controlled by the column fac. The force constant in the
parameter file is multiplied by the value in the column fac for each restraint”

What if the parameter file is missing, then what does fac stand for, force constant? Thanks a lot!

All the best,

Hi Qinghua,

fac just stands for “multiplication factor”, if there is no restraint to multiply this with then you will get a warning of the type “Atom 1 ‘XX’ in moleculetype ‘Other’ is not bound by a potential or constraint to any other atom in the same moleculetype”.

Note that you can define restraints in the [ bonds ] section.

Thanks for your reply @cblau !

I know that distance restraints can be set under [ bonds ] section, but for my case, the distance restraints
are set between molecules, so I have to use [distance_restraints].

Since fac is a multiplication factor, do you know how to feed the restraints to the specific distance? Thanks a lot!

All the best,

I got it, the force constants need to be set in the mdp file!