Seems like the forcefield description of water water.def in TraPPE forcefield folder is not entirely correct:
The attempt to use it in simulations fails with the error:
" Unknown group type (should be 'Flexible' or 'Rigid')"
It is likely because in the file the number of atoms is defined BEFORE the group type (lines 10 and 11). Once they are swapped, the simulations work fine
IDs of the bonds are messed up (on lines 18-19: 0 and 0 instead of 0 and 1, probably). Is it important?
Seems like the forcefield description of water water.def in TraPPE forcefield folder is not entirely correct: