I have also tried it with real units and the results are the same.......In fact, when I used TIP3P water instead of TIP4P, I no longer have this problem: the simulation box does increase, but it eventually decreases and converges to the correct value.
Thus, I can't figure out what went wrong with my usage of TIP4P. I have included a set of my input files using real units.
Vikas, my initial energies upon starting the run are 131.18 kcal/mol
in.swnt_c (1.11 KB)
data.swnt (28.3 KB)