1. WRITE COOR cannot append to an existing file. You can use the CHARMM SYSTEM command to concatenate this file to an existing file, something like SYSTTEM "cat single_focec.dat >> all_forces.dat"
2. Use COOR FORCE COMP. TEST FIRST is a command for testing (and does more than just compute the force, so is slow).
3. The ENERGY command should do an update, which is necessary.

Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden