[gmx-users] Free energy - Hydrogen bond solute-solvent - Ethanol as an example.

[Eudes Fileti]

Hello to all
I have tried to reproduce the hydration free energy (TI) of the ethanol from
Hess and van der Vegt (JPCB, 110, 17616).
The value I have obtained is around 20kJ/mol while the reference value is
-20.1kJ/mol (if not the sign ...).
If someone can help me find the mistake I would be very grateful.
Below are the simulation details.

I followed the protocol of the paper, Berk:
I used 47 lambda values (because of hydrogen bonds between solute-solvent)
(dense near lambda=0 and between 0.46 and 0.72).
I am turning off the LJ and Coulomb terms separately.
Softcore (alpha = 0.5, power = 1), OPLS-AA
Timestep = 2fs,
sd = integrator,
PME
constrained = none
trajectories = 40ps (NVT) (I know that is small, but I looking for
qualitative results) after 20ps of equilibration (NPT).

I have found a value of -4.8kJ/mol for the DeltaG(vacuum) (relative to
mutate the ethanol to dummy in vacuum).
My dv/dl curve (for DeltaG(water) is below. Should I expect this form?
I think there are many "high" positive values. The numerical integration is
15.9kJ.
So, DG(hyd) = DG(wat) - DG(vac) = 15.9 - (-4.8) = 20.7kJ

I believe that this protocol is OK (but I want to confirm that).

@    title "dG/d\8l\4"
@    xaxis  label "Time (ps)"
@    yaxis  label "dG/d\8l\4 (kJ mol\S-1\N [\8l\4]\S-1\N)"
@TYPE xy
0.000   4.431073e+02
0.005   3.660083e+02
0.010   2.987275e+02
0.015   2.023206e+02
0.000   4.431073e+02
0.005   3.660083e+02
0.010   2.987275e+02
0.015   2.023206e+02
0.020   1.905198e+02
0.030   1.086573e+02
0.040   4.217960e+01
0.050   4.101856e+01
0.060   2.314358e+01
0.070   2.223774e+01
0.080   2.669019e+01
0.090   1.696131e+01
0.100   6.089735e+00
0.110   9.569030e+00
0.120   1.505562e+01
0.130   2.785974e+00
0.140   2.440906e+00
0.150   3.050463e+00
0.160   8.578429e-01
0.200   3.577259e+00
0.240  -3.532969e+00
0.280   5.842623e+00
0.320   7.912565e+00
0.360   9.322502e+00
0.400  -2.930754e+00
0.440   2.797944e+00
0.460   5.113900e+00
0.480  -3.919665e+00
0.500  -9.000282e+00
0.520  -7.636168e+00
0.540  -7.574299e+00
0.560  -2.144124e+01
0.580  -6.618514e+00
0.600  -2.755925e+01
0.620  -1.491331e+01
0.640  -2.541398e+01
0.660  -5.213154e+01
0.680  -2.364612e+01
0.700  -2.535817e+01
0.720  -7.767502e+00
0.760  -1.096362e+01
0.800   1.136031e+01
0.840   2.883933e+01
0.880   4.465743e+01
0.920   5.461566e+01
0.960   6.882000e+01
1.000   7.673736e+01

-- 
_______________________________________
Eudes Eterno Fileti
Centro de Ciência Naturais e Humanas
Universidade Federal do ABC
Rua Catequese, 242 - 3º Andar
09090-400 Santo André - SP Brasil
Tel: +55 11 4437-1600 ramal 408
skype: eefileti
http://cromo.ufabc.edu.br/~fileti/
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