Elisabeth, You CAN, in fact calculate the contribution of the reciprocal part of the PME energy to the binding energy between two components in a heterogeneous system, its just quite tedious... say, your system is molecules A and B for which you want to know the interaction energy, and the rest of the system, typically the solvent, we call C. Now your total Reciprocal Coulomb energy will have six parts: ER_tot = ER_AA + ER_BB + ER_CC + ER_AB + ER_AC + ER_BC but these parts are NOT given in the gromacs output as they cannot be calculated DIRECTLY, you have to calculate them by setting the charges on A, B, or C (or combinations thereof) to zero (there is a tool for setting the charges in a tpr file to zero) and then do more runs with: "mdrun -rerun" based on the original trajectory to get the required contributions.
then E_AB = ER_C0 - ER_A0C0 - ER_B0C0 (or something like it, do double check that formula, i can't be bothered thinking it through now ... here ER_A0C0, for example, is the reciprocal part of the coulomb energy with charges in groups A and C set to zero, etc) this being said ... it's tedious, time-consuming, and error-prone (you need to use double precision and save a lot of frames to get reasonably accurate numbers) You might be better off using reaction field, or PME and simply ignore the reciprocal part altogether (if your molecules A, B are NOT charged and have no permanent and large dipole moment you might get away with the latter) What Justin said is correct, PME (or any other Ewald-like method, PPPM, FMA, etc) is standard these days, and for a good reason. However, different properties are affected to a different extent by neglecting the long range interactions, and for what you want to calculate it might be OK for getting at least a qualitative answer, as long as you use PME for the actual MD. (I'd be VERY surprised if everybody who did LIE in the last 10 years went through the trouble outlined above) have fun! mic Elisabeth wrote: > Hello Justin, > > Several days ago you answered my question about calculating nonbonded > terms: > > Question: If I want to look at nonboded interactions only, do I have to > add Coul. recip. to [ LJ (SR) + Coulomb (SR) ] ? > > Answer: The PME-related terms contain both solute-solvent, > solvent-solvent, and potentially solute-solute terms (depending on the > size and nature of the solute), so trying to interpret this term in some > pairwise fashion is an exercise in futility. > > my question is if I want to add up nonbonded related terms to get inter > molecular energies, do I have to add Coul. recip. or it is already > included in Coulomb (SR)? > They are separate energy terms. The PME mesh terms is "Coul. recip." and the short-range interactions (contained within rcoulomb, calculated by a modified switch potential) are "Coulomb (SR)." > and also, for a A-B system, I have been using energy groups to extract > solute-solvent, solvent-solvent, solute-solute terms. Did you mean that > applying doing so with PME as electrostatics treatment is not correct? > PME has been consistently shown to be one of the most accurate long-range electrostatics methods and is widely used, but in your case is preventing you from extracting the quantity you're after (if it can even be reasonably defined at all). Using energygrps will not resolve the problem I described above. The "Coul. recip." term contains long-range energies between (potentially) A-B, A-A, and B-B, depending on the nature of what A and B are. The only terms that are decomposed via energygrps are the short-range terms, which are calculated pairwise. Thus, with PME, there is no straightforward way to simply define an "intermolecular energy" for a heterogeneous system. You might be able to define such a term for a completely homogeneous system (which also assumes that the sampling has converged such that the charge densities etc are uniform...but I'm sort of thinking out loud on that), but not one that is a mixture. -Justin > Thanks for your help! > Best, > > > -- -- gmx-users mailing list gmx-users@gromacs.org http://lists.gromacs.org/mailman/listinfo/gmx-users Please search the archive at http://www.gromacs.org/Support/Mailing_Lists/Search before posting! Please don't post (un)subscribe requests to the list. Use the www interface or send it to gmx-users-requ...@gromacs.org. Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
