[gmx-users] about dg/dl and benzene solvent

[acorrea at unisa.it]

Hallo to everybody.

I have same problems with free-energy perturbation (thermodynamic integration)
calculations using benzene as solvent.

When I do a simulation of just benzene (ONLY SOLVENT!!!) and I switch on free-
energy option in the file.mdp,I found non zero values for dg/dl, without any
definition of lambda in the topology ...

The values of <dg/dl> are different for different values of init-lambda
(between 0 and 1). This makes no sens to me, and makes my TI calculations
garbage.

As a test, I tried with other three pure solvents: water-spc , methanol and
dmso (they are in the tutorials of gromacs). When I switch on the free-energy
option with these three solvents I always obtained dg/dl = 0 (which makes
sense to me).

I tried to check for differences in the topol.tpr (by  gmxdump) but I did not
find anything helpful...

I am confused because in all cases I don't define lambda. Why just for benzene
I found non zero values for dg/dl ?

I imagine I've some problems with my benzene topolgy, but I can't understand
what. Incidentally, before discovering this problem I run NPT simulations of
my benzene, and all the properties (density ecc ecc) are perfetctly reasonable.

I attach the topology of benzene, and the file.mdp ..

Thank in advance for  any suggestions
best whishes  
Andrea


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