Hi, I noticed that in the spc.itp and spce.itp of GROMACS, the OH bond force constant is 345000 kj/(mol nm2). However, in LAMMPS, this constant is 553.935 kcal/(mol A2). If it is converted to GROMACS unit, it will be around 463533 kj/(mol nm2). I am wondering why there is a difference between GROMACS and LAMMPS? Thanks!
SPC and SPC/E models have no bond force constants as the models are rigid. The only reason to have a bond potentials in files for those models is to allow methods that do not support constraints to work, in particular energy minimization.
Thanks. Is it the same for the angle parameters below the bond parameters that they only function if water molecules are defined as flexible?
Indeed.