Why the Coulombic pairwise energy is so different for two kinds of rigid water implementations- fix shake and fix rigid

Dear Steve,
Thank you for you prompt reply.
In both models, I didn’t specify the special_bonds command.
In fix shake model, bonds and angles are constrained by fix shake command. In the fix rigid model, bonds and angles are not specified in data file, since I think they are unnecessary for rigid bodies.
The thermo output for ebond and eanlge are all zero during the simulations. evdwl on step 0 are almost the same (9503.941 kcal/mol vs. 9503.9436 kcal/mol).

Special bonds are on and exclude neighbor interactions by

default for bonded atoms.

If you don’t turn off pairwise interactions within the rigid

bodies (see neigh modify exclude molecule) you

will compute them for rigid bodies (no bonds), but not

for fix shake (bonds with exclusions). That could

be the difference in your energies. The intra-body

forces cancel, so it shouldn’t affect dynamics.