Dear List,
I'm still trying to simulate SPC/E water in Lammps 
I'm now finding that the E_long value seems too high. Using NAMD I
get the configurational energy of my water to be about -45 kJ/mol,
which is what I expect. In LAMMPS the VDW energy seems correct, and
the Coulombic energy is in the ballpark, but the E_Long is much higher
than I would expect it to be. I'm attempting to use PPPM
I've put my input files here:
http://gubbins.ncsu.edu/users/joshua/lammps_question
Am I making a mistake in the implementation of the EWALD/PPPM?
Thanks in advance. Below is a comparison of values between NAMD and LAMMPS.
Josh
E_Coul (LAMMPS): -659.8 eV
E_Long (LAMMPS): -3,069.6 eV
E_VDW (LAMMPS): 105.0 eV
PotEng: (LAMMPS): -3,624.3 eV
Configurational Energy (per 1267 water molecules): -2.86 eV or -276 kJ/mol
E_Coul (NAMD): -694.2 eV (with PME)
E_VDW (NAMD): 105.0 eV
PotEng (NAMD): -589.2 eV
Configurational Energy (per 1267 water molecules): -0.465 eV or -44.9 kJ/mol