About the sigma adjustment parameters in charmm2lammps.pl

Hi all,

Sorry for the global message. I hope the message can help clarify the usage of charmm2lammps, or please correct me if I am wrong.

I recently need to convert .psf and .pdb files to LAMMPS data file, which was easily done thanks to charmm2lammps.pl. However, I found that the adjustment parameters for sigma of Lennard-Jones potential is set to be 2**(5/6), which might not apply if I convert from the parameter file used for NAMD instead of for CHARMM. Here is the reason: is the 2**(5/6) parameter relevant with a sixth power mixing rule? But both NAMD, as well as the LAMMPS input file generated directly from charmm2lammps.pl use arithmetic mixing rules for pair potentials, which points to a adjustment parameter of 2. Does CHARMM program use sixth power mixing, where the 2**(5/6) power originates from?

I am worried because I got 10% difference in bulk water density between NAMD and LAMMPS in the same thermodynamic conditions. Thank you in advance if you could help me with the question.

Best regards