I want to thank you for all your helpful suggestions. I have tried each and every but to no avail.
Basically, I want to perform an NPT analysis uing TIP4P water model to determine if I can get back the density of water at 300k 1 atomsphere. However all my attempts to do so have failed, and the volume of my box eventually becomes exceedingly large (from 3000 A3 to something like 10^8 A3), and fluctuates betweeen (20000 A3 and 10^8 A3). My initial box setup already has the correct density of water.
I have tried the same computation using TIP3 and it works perfectly.
Can any user suggest why? If someone has done it before (combined TIP4P and NPT), could they post a sample input file?