I want to use pair_style lj/cut/tip4p/long along with compute group/group command. But the following error popps up:
Pair style does not support compute group/group
I am wondering if there is an alternative method or not?
No. This is because of the ambiguity between point M and the oxygen atom in the special processing of interactions in tip4p styles. That is the price you pay for the improved computational efficiency.
You would have to model TIP4P with 4 explicit sites (using a tiny nonzero mass for the M point since LAMMPS doesn’t allow massless atoms) and fix rigid/small instead of fix shake. Then you can use pair style lj/cut/coul/long which does support compute group/group, however this comes with a significant drop in computational efficiency due to the increased number of interaction sites and having to use a much smaller time step for fix rigid to maintain proper energy conservation.