[gmx-users] soft-core and coulomb transformation

[bharat v. adkar]

Dear all,
   i am trying some very basic transformations by thermodynamic integration 
to calculate free energy difference. By following mailing-list archive, i 
learned that it is efficient to do coulomb transformations and LJ 
transformations separately. i use David Mobley's tutorial values for 
soft-coring, i.e., sc-alpha=0.5, sc-power=1, sc-sigma=0.3. As per list, 
since charge transformations are not "hard", we are at liberty to use 
soft-core potentials for coulombs and that is quite logical, and yes, 
without sc, there is no problem in coulomb transformation.

Everything goes fine till this point. Now, i do reverse transformations to 
check for hysteresis. For vdw transformations, i get almost perfect 
overlap of dG/dlambda vs lambda for forward and reverse transformations, 
but for coloumb transformations, if i don't use soft-core potentials, i 
don't get any overlap (i understand that getting overlap in forward and 
reverse transformations is not the accurate-enough measure of absence of 
hysteresis, but for smaller systems 5 ns at each lambda value should be 
sufficiently enough sampling).

Ny doubt is why should this happen? if there is no problem like 
singularities in potentials, why should not using sc-potentials make 
such a difference?

i tried this for simple ala->gly and ethane->methane transformations.

please give me some insight into the problem...

bharat

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