Alchemical change of charges biases Coulomb-14 evaluation in log-file
I've performed an alchemical change of state of Aspirin in TIP3P within Gromacs 5.1 where I did change the partial charges of the atomtypes.
Although the two sets of partial charges differ significantly, the average Coulomb-14 interactions as provided in the log-files of the alchemical end states are nearly identical.
I repeated the alchemical change of state with fixed partial charges of typeA and typeB respectively. Those Coulomb-14 interactions differ significantly due to the influence of the charges.
The interactions of the simulation with a change of charges tend to represent the behaviour of the charge set of topology A (lambda = 0) in both end states. This seems odd, since in the alchemical end state B (lambda = 1) type B charges should be used.
Add check for double perturbation
Added a check for changes atom charges and/or types through both
the topology and the couple-moltype option. The couple-moltype
option would override the topology settins for decoupled states.
#2 Updated by Berk Hess over 4 years ago
- Category set to preprocessing (pdb2gmx,grompp)
- Status changed from New to Accepted
You are changes the charges both manually in the itp file and "automatically" by setting couple-moltype = ASP in the mdp file. I guess that removing the couple-moltype option will do what you want.
We should add a check that the top file doesn't perturb any atoms when using couple-moltype.
#5 Updated by Andreas Mecklenfeld over 4 years ago
thanks for your answer. When I remove the couple-moltype option, the free energy difference of the ligand in bulk with different charge sets will be calculated, correct?
My original idea was to treat the alchemical end states differently in order to describe the ESP of the ligand in bulk and vacuum with corresponding charges. Is this not possible?
#6 Updated by Berk Hess over 4 years ago
These are user questions that should be posted to gmx-users, please post there if you want to continue this discussion.
Yes, you will calculate the free-energy difference of the different charge sets. But note that you are also changing intra-molecular interactions (1-4 and longer distance), so it's not obvious what the meaning of this free-energy difference is.
I don't see the point of using different charge sets for bulk and vacuum, since the force-field does not give proper energy differences for changing charges (this is related to my first comment). If you want to correct for changing charges, you should use solvation optimized charges and calculate the polarization correction for the change of polarization between vacuum and solvated using an electronic structure method. You should search the literature for this if you're interested in doing that.