[gmx-users] How to extract extract dV/dlambda data with g_energy??

[PeiQuan Chen]

Dear gmx-users：
  I have done a FEP calculation on the hydration free energy of methanol as been done by Alessandra villa in the JCC 2002(23),548-553.
  I have set ionit_lambda to a distinct number of values(say 21, 0,0.05,0.10 ... 1.0) and
the delta_lambda to 0.0(Method TI).  After MD(200ps) on one points have have done, I use the g_energy program to extract the dv/dlambda from the ener.edr files.
  The command I use is:
  g_energy -f ener.edr -o free.xvg -b 100
  One of the result(lambda=0.1) is the:

Statistics over 50001 steps [ 100.0000 thru 200.0000 ps ], 1 data sets

Energy                      Average       RMSD     Fluct.      Drift  Tot-Drift
-------------------------------------------------------------------------------
dVpot/dlambda               84.7103    64.1511    64.1027  -0.086365   -8.63667  

  Because the dVpot/dlambda is so high(84.7103), and the fluct. is high too(the calucation
hydration free energy value have bone by Alessandra villa is -14.1 KJ/mol). I suspect It been some wrong I have been done. I think it maybe something wrong I have used the g_energy program. Can anyone help me to correct it? 
  
  Thank you in advance!


Sincerely yours
---- 
PeiQuan Chen
________________________________________________________________________
PeiQuan Chen 	Department of Chemistry, Nankai University
WeiJin Road 94, TianJin, 300071, P.R.China
phone:	(86)2223506728		Mobile Phone: (+86)13920675030
gromacs at 163.com	
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