[gmx-users] heat of vaporization

[Saitama Lawliet]

Hi all

I am trying to calculate heat of vaporization of tri-ethyl phosphate (tep).
I took 500 tep molecules for MD simulation. After energy minimization and
NVT I performed NPT simulation for 10ns with usual PBC, dispersion
correction etc.
>From the final results using g_energy -nmol 500 I got the potential energy
of -155.406kj/mol.
The density  of the liquid tep was 1085.8kg/m3 which is very close to the
earlier MD reported data.
Then I took one single tep molecule in a box and perform the calculation
without using pbc and all cut-off to zero. The potential energy I got was
-692.13kj/mol.

Now as per defintion of HVap= U(gas)- U(liquid) +RT
 I am getting the value of Hvap of tep as: -534.25kj/mol whereas the
reported value is 77.69kj/mol. what is wrong I am unable to find out.
Any help to sort out this mistake will be highly appreciated.

warm regards

APJ