Subject: Some questions on the QEq charge model

From: Guosheng Wu <wu_guosheng2002/at/yahoo.com>
Date: Mon, 1 Nov 2004 19:09:13 -0800 (PST)

Hi there,

I wonder if there is anyone in this list familar with the QEq charge
model.
 
Recently I am reading the original QEq paper, and like to post some
questions about on the CCL. Hope I can get some help from you.
  
The attached paper: "Charge Equilibration for Molecular Dynamics
Simulations", by A. K. Rappe & W. A. Goddard III, J. Phys. Chem. 1991,
95, 3358-63.

Q1: There seems to be some typo on the equations in the above JPC
paper: equation (10) should be CQ=-D, instead of CD=-D(like a
eigenequation?); and C1j=1, instead of C1j=Qj. One may claim that the
original equations may still hold with more sophisticated derivations,
similar to some QM equations, but it will be nice to see how to get to
those equations. BTW, some references on the paper marked as "to be
published" seem to be not ever published. Are there any good reasons?

Q2. If you have implemented it, or ever applied it from commericial
package(UFF or Cerius2 ?), is there any good sense of its performance?
In principle, this model should be more accurate than a similar but
simpler Gasteiger model. The relative speed may be one issue since there
can be heavy computations needed to get the inverse of a big matrix. Are
still any benchmark study on the comparison. Not to I know with some
search over the literature. Maybe there is no good creteria on charge
models since they are not experimentally measurable, and simulation
results may be coupled with other issues. Even it's not quite clear from a
recent paper, PNAS(2002,99,12622) on GPCR and docking, where the charge
for protein was generated either from CHARMM22 or from the QEq method. It
is likely the only information on QEq one can find from that paper.

Q3. I did a little search over google with "CCL, QEq" as keywords.
Some hits came out. There seem to be lots of questions, but few good
answers. Just list a few:
 
  http://www.ccl.net/cgi-bin/ccl/message.cgi?2002+09+23+009 (even in
Gaussian)
  http://www.ccl.net/cgi-bin/ccl/message.cgi?1998+01+23+002
  http://ftp.ccl.net/cgi-bin/ccl/message.cgi?1996+01+10+008
  http://www.ccl.net/cgi-bin/ccl/message.cgi?1995+12+17+001
 
Anyway, I look forward to your comments on these questions. Any kind
of information will be highly appreciated.

Best regards,
 
Guosheng Wu

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