Subject: CCL:M:M: esp failure

From: [email-masked] (Ross Nobes)
Date: Fri, 7 Jun 1996 15:52:45 +0100 (BST)

A J Turner writes:
  > I have been approached by a couple of people over this mopac deck
  > that caused some problems for me. I include the .dat and .out files.
  > My concern is that the C to which the Cl is bonded has a charge (in
  > the esp calc) which is highly negative whilst the Cl has a small
  > positive  charge.
  > 

Using Alex's final structure, I find the following ESP charges:

Ab initio HF/6-311+G(d)

                 CHelpG       CHelp       Merz-Singh-Kollman
C                 0.17         0.54            -0.07
Cl               -0.23        -0.28            -0.20

Semiempirical

                  AM1          AM1            PM3         MNDO
                           (dipole cons.)
C                -0.74        -0.54          -0.44        -0.31
Cl                0.00        -0.08          -0.05        -0.09

So, depending on which method one uses, the charge on C can range from
+0.5 to -0.7! Could anyone please tell me whther there have been any 
recent reviews on determining "good" ESP-derived charges for use in, for
example, force-field based methods?

Cheers,
Ross Nobes

-- 
+----------------------------------------------------------------+
| Dr Ross Nobes                                                  |
| Senior Scientist, Quantum Mechanics                            !
|                                                                |
| Molecular Simulations                                          |
| 240/250 The Quorum                 Phone:  + 44 1223 413300    |
| Barnwell Road                      Fax:    + 44 1223 413301    |
| Cambridge CB5 8RE, UK              E-mail: [email-masked] |
+----------------------------------------------------------------+