I would call a NPT simulation a simple relaxation calculation. If Volume, number of particles and Energy are both fixed there will be transfer from kinetic energy to potential energy and vice versa and no relaxation is my understanding. So why would you call it a relaxation calculation?
If you do a true relaxation (using NPT with PBC) OR use the minimize command (which probably does not need PBC - check the docs please) AND THEN do a NVE simulation perhaps the atoms will not fly off.
Best Regards,
Manoj
2009/6/5 Jarvis Chong <zhuanghl@…33…24…>