[gmx-users] Unexpected free energy values.

[sujithkakkat .]

Dear all,

  I am studying the CH4-CO2-H2O mixture at low T(270K) and high P (20 bar).

Forcefields used: CH4 : Single point LJ site; CO2 : EPM2 (Rigid with two
virtual sites); H2O: TIP4P.

I computed the free energy of solvation for a CH4 molecule in  H2O in the
presence of CO2 at different concentrations. The BAR method was used for
the calculation. I decoupled the CH4 molecule in 20 steps with lambda
varying by 0.05 each step.

Experiments have revealed that CO2 and CH4 are cosolvents for each other in
water and CH4 solubility will increase in presence of CO2.

However, I am getting more positive value for the free energy of solvation
of CH4 when the CO2 concentration is increased. Based on the experimental
data one would expect that the free energy becomes less positive (more
negative) since CO2 was observed to enhance CH4 solubility.

The forcefield combination used here was used earlier for studying
CH4-CO2-H2O system, though in a different context where the authors
simulated hydrate formation. So I guess the morels used are fine.

I wonder whether the problems lies in my MDP file . The free energy
calculation parameters used are given below.

; Free energy control stuff
free_energy             = yes
init_lambda              = 0.0
delta_lambda           = 0
foreign_lambda         = 0.05
sc-alpha                   = 0.5
sc-power                  = 1.0
sc-sigma                  = 0.373 ; equal to sigma of the single point CH4
model.
couple-moltype         = CH4
couple-lambda0         = vdw
couple-lambda1         = none
couple-intramol          = no
nstdhdl                      = 10


Any comment will be of great help.

Regards,

Sujith.