Dear:
Recently I found the following statement from CHARMM website
(
http://dogmans.umaryland.edu/~kenno/cgenff/faq.html#esp):
/I obtained Mulliken, NPA, AIM, Hirschfeld (aka stockholder) or
Merz-Kollman (or CHELP or other (R)ESP fitted) charges. Can I use
them as they are?/
/No! Charges that are directly derived from the wavefunction (such as
Mulliken, NPA, AIM, Hirschfeld,...) are generally considered incapable
of reproducing accurate electrostatic interaction energies in the
framework of an atom-centered point charge model (as used by Class I
additive force fields such as CHARMM) without emiprical corrections such
as the "BCC" scheme. Force fields that do use unmodified charges based
on vacuum QM calculations typically use an indirect method, fitting the
atomic charges to reproduce the ElectroStatic Potential (ESP) at many
points around the molecule. However, //*CHARMM falls into neither of
these categories*//. For charges to be compatible with the L-J (and
dihedral) parameters in CHARMM, they must be optimized based on
interactions with water, as described in the CGenFF paper and tutorials.
The Merz-Kollman charges mentioned in said paper and in the old tutorial
are //*only*//meant as an easy way to obtain an //*initial guess*//, and
have been superseded in this role by the CGenFF charges produced by
//the CGenFF program
<
http://dogmans.umaryland.edu/%7Ekenno/cgenff/program.html>//. Same for
other (R)ESP methods and AM1-BCC and the likes; they may be appropriate
for other force fields, but not for CHARMM. Different force fields are
based on different parameterization philosophies, and all parameters are
interconnected and have to be consistent. If you don't have the time or
resources to perform charge optimization based on interactions with
water, it is strongly recommended to assign charges by analogy to
existing CGenFF compounds, which were properly optimized. In fact, //the
CGenFF program
<
http://dogmans.umaryland.edu/%7Ekenno/cgenff/program.html>//does this
in an automated fashion, and is presently the recommended source of
initial guess charges (and parameters); see //this related FAQ entry
<
http://dogmans.umaryland.edu/%7Ekenno/cgenff/faq.html#procedure>//. /
It seems that we cannot use RESP charge for CHARMM FF? However, when we
read the paramter comments for RED-VIII.5
(
http://q4md-forcefieldtools.org/REDS/popup/popkeyword.php), we can find
the following:
/ -6- *ESP-A1*
HF/6-31G*//HF/6-31G* - Connolly surface algo. used in MEP computation -
1 stage ESP fit qwt=0.0
Used in some AMBER, OPLS & CHARMM force field based simulations //
//
//// -7- //*ESP-C1*//
//HF/6-31G*//HF/6-31G* - CHELPG algo. used in MEP computation - 1 stage
ESP fit qwt=0.0/ /
//Used in some OPLS & CHARMM force field based simulations/
So, Iam quite confused for such different comments. Does anybody have
any idea for this?
thank you very much.
best
Albert
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Received on Wed Nov 28 2012 - 01:00:02 PST
