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.