Since there are no responses, here is a “smaller” question, and once again, I think it was discussed here. Is it possible to specify a fudge factor that is not equal to the ff default for particular 1-4 pairs without writing out funny charges or new modified LJ parameters?
Example: we have a sheet of hexagonal boron nitride (hBN) and the atoms are massively charged (+0.9 and -0.9 for boron and nitrogen, respectively). Under OPLS-AA with its fudge of 0.5, the whole thing is perfectly stable. When the parameters are copy-pasted into CHARMM (generally a highly questionable idea, I know), it distorts. However, when I rescale all the Coulomb charges by 1/sqrt(2) (no change to LJ), everything is back to normal for the hBN sheet. This to me is a good indication that the distortion is mainly due to CHARMM’s default 1-4 factor of 1.0 applied to Coulomb.
I would like to emulate the majority of what kept the hBN’s geometry in the original ff – it is perfectly sufficient for our purposes. Hence, is it possible to cleanly specify a fudge factor of 0.5 for B-B, N-N, and B-N pairs so that it will selectively override the default of 1.0?
Also, our situation is a bit different from that described here. We have edge atoms, which will have charges different from the +/-0.9 mentioned above, while the atomtypes are the same as for the ones with charges +/-0.9. What we want is that the fudge factor is 0.5 for whatever the charges are, as listed in the atoms section of the topology.
Thanks!