David,
It still seems strange, if one can get a lot of other properties
correct, like DHvap, dielectric constant, density, diffusion constant etc.
Unfortunately my simulations are converging very slowly, but I hope to
have more conclusive results soon.
Ah, you checked all those things? I suspect in my case I would get
incorrect DHvap, etc depending on my charge set.
Yes, I would expect these to be pretty slow to converge: You're
dealing with a pretty big molecule. I run 5 ns for relatively small
molecules that don't even have any rotatable bonds.
Reasonable Estimate of Simple Point charges...
It's not that I know of any better method to do it, but the QM levels of
theory typically used for computing charges are *very* low, and far from
convergence to the basis-set limit. In addition RESP is not a good
method, because the problem is underdetermined... You know, Peter
Kollman once told me that they did quantum calculations followed by RESP
and then multiplied the charges by 0.9.
So we have a preprint on this. We've done about 8 different levels of
QM going up to MP2/cc-PVTZ and B3LYP/cc-PVTZ, with and without
reaction field treatment of solvent.
I agree that RESP is not ideal, but I haven't seen anything that works
better. If you have, I'd love to hear about it. We're not, however,
doing anything strange like multiplying by 0.9.
<cynical>
Or: for interesting molecules you're out of luck, unless you're lucky
</cynical>
Exactly. :)
Or, in other words, we should worry about this far more than we do.
David
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