[gmx-users] Potential energy in Zwanzig relationship

[gozde ergin]

Dear all,

I run thermodynamic integration simulation with gromacs and got the free energy by g_bar command.
I also would like to estimate this free energy by using Zwanzig relationship of \DeltaG = -RT ln (<exp{-\Delta U_(ij)/RT}>_i 
Here U is the potential energy, right? 
However the results that I am getting with g_bar (82 kcal/mol) and with Zwanzig equation (406 kcal/mol) are totally different. 
Do anyone has an idea why I am getting that different results?

Bests