On 9/11/13 6:07 PM, Rafael I. Silverman y de la Vega wrote:
Hi all,
in another thread it was recommended that instead of changing the atom
types by analogy to ones with the desired parameter already assigned, that
instead one should assign bonded parameters by analogy. I was just
wondering how this is better?
I didn't suggest that. I said that any assignment by analogy can have negative
outcomes. Bonds and angles, in theory, are a bit easier to deal with because
the nonbonded interactions between their atoms are excluded. Torsions are very
difficult because the torsional bonded terms have to be balanced against the
charge and LJ parameters of atoms separated by 3 bonds, i.e. 1-4 interactions.
Let me state very clearly that, for any missing parameters, anything short of a
thorough parametrization and validation is (in my purist mind) unsound.
As long as the atom type isnt too different, then why would the torsions
not work out right? If the parameters are interrelated, what makes it
better to add one term to an atom type rather than changing all the terms
(to ones that are supposed to be together)?
The key is being sufficiently similar, but even subtle differences between LJ
parameters have very large consequences, especially for 1-4 interactions, so I
find it hard to believe that one will recapitulate accurate interactions in this
case. Of course, all assignments by analogy bear some penalty for nonequivalent
groups. I balked at your initial post in the other thread because there was no
need to mess with atom types. The system at hand there contained protein and
lipid and could be completely described by existing atom types and bonded types,
the OP there was just (and perhaps still is) constructing the force field
incorrectly. I felt it need to be stated that the approach there would not
necessarily be productive, and to make sure the archive was complete so that no
one would come across a post in vacuum and assume that he or she can simply swap
around atom types to make error messages go away. Your initial statement
referred only to bonds and angles, which indeed would be insensitive to changes
in LJ parameters on any of those atoms, but since the thread was in the context
of dihedrals, I thought I should state my opinion there.
-Justin
--
==================================================
Justin A. Lemkul, Ph.D.
Postdoctoral Fellow
Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 601
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201
jalem...@outerbanks.umaryland.edu | (410) 706-7441
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