Dear all,

I'm attempting to run NVT on water. The molecules evaporate and leave the simulation cell in spite of negative potential energy of system. Does negative potentials imply attractive forces among molecues?

Please notice the images below displaying frame 1 and last frame, the energy profiles along with the MD paramters. Does this have to do with periodic boundary conditions (that could be missing?)? Where do I find PBC options? Your comments and insights will be highly appreciated. 

Many thanks

---- Dynamics parameters ----

Ensemble : NVT
Temperature : 298.00 K
Control method : Nose
Q ratio : 0.0100000
Timestep : 1.00 fs
Number of steps : 1000000
Duration : 1000 ps
Initial velocities : Random
Random number seed : 1643353105

---- Energy parameters ----

Forcefield : COMPASSIII (Version 1.1)
Charges : Forcefield assigned
Electrostatic terms:
Summation method : Group based
Cutoff distance : 15.5 A
Spline width : 1 A
Buffer width : 0.5 A

van der Waals terms:
Summation method : Group based
Cutoff distance : 15.5 A
Spline width : 1 A
Long range correction : YES
Buffer width : 0.5 A

---- Thermostat ----

Nose mass parameter, Q mass: 5.31192 kcal/mol*ps^2

---- Dynamics summary ----

Initial Final Average Std. Dev.
------- ----- ------- ---------
Tot. energy (kcal/mol) 329.988 -623.105 -582.035 23.635
Pot. energy (kcal/mol) 64.392 -845.692 -847.631 18.488
Kin. energy (kcal/mol) 265.596 222.586 265.596 13.215
Tot. enthalpy (kcal/mol) -1327.516 -626.522 -541.144 128.118
Temperature (K) 298.000 249.743 298.000 14.827
Pressure (GPa) -3.850 -0.008 0.095 0.290
Volume (A^3) 2991.461 2991.461 2991.461 0.000
Density (g/cm^3) 1.000 1.000 1.000 0.000