Hi Lammps Users

I am using COLVARS module to perform umbrella sampling in a LJ system. I am using “units lj” for all LAMMPS simulations. I have one quick questions:

When I give the input value of spring constant, should it be in kBT/sigma^2? I am performing WHAM analysis after this using the Grossfield WHAM where I have to input data for the spring constant. To be consistent, I just want to make sure that LAMMPS do use the units in kBT/sigma^2. While going through the module (reference manual) I see it as colvar unit - but could not find a precise definition for the same (may be I might have missed it). Any help would be greatly appreciated.


Vaidyanathan M S

Hello Vaidyanathan, the units used in Colvars are those chosen in your LAMMPS run, i.e. for force it’s epsilon/sigma with units lj.

“colvar unit” in the reference manual indicates that different types of variables represent different physical properties (distances, angles, etc) and even after choosing a consistent unit system the unit of the variable depends on its type, which is chosen by you.

The force constant in the harmonic restraint can be optionally scaled by the “width” option so that the same restraint can be applied to different variables. If you left “width” at its default value of 1, the force constant is the number that you entered in the input: otherwise, its scaled value will be reported at definition time for use in e.g. WHAM.