RE: CHELPG

From: "Shobe, Dave" <dshobe %-% at %-% sud-chemieinc.com>
Subject: RE: CHELPG
Date: Tue, 09 Jul 2002 15:11:18 -0400
Mulliken charges are assigned to atoms based on the "usage" of
 the atoms'
 basis functions in the molecular orbitals that are calculated.  It is known
 as a quick-and-dirty way of assigning charges, but one that doesn't
 necessarily have any physical meaning.  Mulliken charges are notoriously
 basis-set dependent.
 CHELPG charges are based on the electrostatic field generated by the
 molecule.  It is a best-fit method; the set of charges produced by the
 CHELPG method are the point charges that would produce an electric field
 which is as similar as possible to the molecule's.  Accordingly, if you need
 the charges for molecular-mechanics calculations which include electrostatic
 interactions, CHELPG is arguably the best for that purpose.
 There are other methods of assigning atomic charges.  GAPT charges are based
 on the derivative of the molecular dipole moment WRT the atomic positions.
 AIM charges are based on the division of 3-dimensional space among the atoms
 in the molecule, according to a surface that has certain mathematical
 properties.  NBO charges are similar to Mulliken charges, but the orbitals
 are transformed into "natural" orbitals, which fixes some of the
 basis-set
 dependency of the Mulliken method.
 --David Shobe
 Süd-Chemie Inc.
 phone (502) 634-7409
 fax     (502) 634-7724
 email  dshobe %-% at %-% sud-chemieinc.com
 Don't bother flaming me: I'm behind a firewall.
 -----Original Message-----
 From: George Hall [mailto:georgehalli %-% at %-% hotmail.com]
 Sent: Tuesday, July 09, 2002 10:21 AM
 To: chemistry %-% at %-% ccl.net
 Subject: CCL:CHELPG
 Hi everybody,
 I wonder if some could point me out what are the differences between Chelpg
 charges and Mulliken.
 Would you please indicate me some references or some advices about the
 aproach for these kind of charges.
 Thanks a lot in advance.
 George Hall.
 _________________________________________________________________
 Chat with friends online, try MSN Messenger: http://messenger.msn.com
 -= This is automatically added to each message by mailing script =-
 CHEMISTRY %-% at %-% ccl.net -- To Everybody  | CHEMISTRY-REQUEST %-% at %-%
 ccl.net -- To Admins
 Ftp: ftp.ccl.net  |  WWW: http://www.ccl.net/chemistry/   | Jan: jkl %-% at %-%
 ccl.net