CHARMM Development Project Forums User Discussion & Questions Molecular Dynamics Water Molecule occupancy at a position?

Analysing hydrogen bond patterns for:
#donors #acceptors
1st sel: 767 1744
2nd sel: 0 0

For this command to work, both atom selections need to have either donors or acceptors (or both); the 2nd selection is clearly a problem (0 for both), hence the error message.

Your habit of posting cryptic excerpts out of context makes it difficult to say much more.