I was looking at how to use LAMMPS to calculate potential energy surface . And I have found that there is a way to calculate PES using fix restrain method (http://lammps.sandia.gov/doc/fix_restrain.html).
I was testing this method to compute PES of ethane (dihedral H-C-C-H). But the result does not look right. I mean the maxima should be at 0, 120 … dgree and minima should be at 60, 180 …degree. However my result shows maxima at 60, 180 and minima at 0, 120.
FYI, I used GAFF, low temperature 0 high temperature 600 restraint coeffs 0 and 3000, angle for 0 to 360.
This has nothing to do with fix restrain or PES. It is about how to define the particular dihedral style. You don’t provide any details on how you are specifying the dihedral style or how you are extracting the dihedral energy from LAMMPS. There are many ways to do these things and many ways to get them wrong. I suggest you focus on a couple of simple geometries (e.g. trans, gauche, cis) and make sure you are correctly reproducing the known numerical values of these, not just the correct max/min feature. After that you can use fix restrain to scan the entire PES.