Subject: Calculation of solvation free energy for methane???

From: "Asif Rahaman" <Arahaman<<at>>chemdept.chem.ou.edu>
Date: Sun, 1 Feb 2004 06:38:08 -0600

Dear CLLers:

     I am trying to calculate the solvation free energy of methane in water
using Gaussian 03.
I have used PCM model with Radii = UAHF and SCFVAC option. I get the
following output:

 Variational PCM results
 =======================
 <psi(f)|   H    |psi(f)>                     (a.u.) =     -40.195171
 <psi(f)|H+V(f)/2|psi(f)>                     (a.u.) =     -40.195260
 Total free energy in solution:
  with all non electrostatic terms            (a.u.) =     -40.184161
 --------------------------------------------------------------------
 (Polarized solute)-Solvent               (kcal/mol) =       -.06
 --------------------------------------------------------------------
 Cavitation energy                        (kcal/mol) =       9.98
 Dispersion energy                        (kcal/mol) =      -3.07
 Repulsion energy                         (kcal/mol) =        .05
 Total non electrostatic                  (kcal/mol) =       6.96

-----------------------------------------
Now (Total free energy in solution: - <psi(f)|   H    |psi(f)>)*627.51 =
6.90 kcal/mol
This is the solvation free energy I get for methane (am I right???). But in
the ref.(289 in G03 manual)
J. Chem. Phys. vol.107  page 3210, 1997 by Barone et al. reported for the
same is 1.86 or
1.83 kcal/mol.

I do not know where I am doing wrong that I can not reproduce their results.
even the calculated (mine)
cavitation, electrostatic and dispersion - repulsion contributions to the
total solvation free energy
change are not comparable to their results ( see table 4 in ref 289).

Can anybody please help me reproducing the results or pointing out my
mistake so that I can get right results.
By the way, the experimental value for the solvation free energy for methane
is about 2.0 kcal/mol.

I am including my input file in case you need details of my calculations.

-----------------------------------------
$ RunGauss
%NProc=4
%chk=ch4
%mem=64MB
#p HF/6-31g* opt freq pop=chelpg scrf=(pcm, solvent=water)

Methane B3LYP/6-311G**

0 1
C1
H1 1 b1
H2 1 b1 2 a1
H3 1 b1 2 a1 3 d1 1
H4 1 b1 2 a1 4 d1 1

b1 1.1
a1 109.5
d1 109.5

SCFVAC
VWD
EPS=78.5
RADII=UAHF
DENSITY=1.000
TABS=298.15