I have run the 0.9 ethanol and 0.1 water system. the experimental density is 0.799 and I initiate the simulation from 0.760. First I run the NVT the the NPT, but in the NPT the volume increse and density goes to zero, while density must approach to 0.799. Can anyone help? The pcff force field have been used and the simulated result have been attached.

Is it possible that the tottal energy of the system be negetive