MNDO ESP-Charges

[rwoods - at - biop.ox.ac.uk]

 Dear Netters,
 In reply to Joe Leonard's question regarding ESP-charges derived from an
 MNDO wavefunction, we have recently examined the use of such charges in a
 force field for carbohydrates.  We looked at the ability of these charges
 to reproduce hydrogen bond energies and dipole moments.  Scaled MNDO ESP-
 charges reproduced the HF/6-31G* hydrogen bond energies within about 0.5
 kcal/mol.  (Work submitted to JACS)  Previously we found that HF/6-31G*
 ESP-charges could lead to an excellent reproduction of the SCF-dipole
 moment (J.Comput.Chem 11 (1990)297-310) so we had a look at the MNDO case.
 We and others (Breneman and Wiberg, J. Comput. Chem. 11 (1990) 361) have
 noted that charge convergence requires that the ESP should be sampled at
 a surprisingly high density of points.  As far as where the points need to
 be located, we have found that as they should be between the VDW surface
 and about 3 angstroms of it.  In our recent work with MNDO we modified the
 WILLIAMS surface in MOPAC_6.0 such that the default GRID spacing was
 decreased from 0.8 to 0.5 angstroms and extended the sampling shell from
 1.2 to 2.8 angstroms.  Such an arrangement produces about 750 points per
 atom.  We also compared the results with a random point location method
 filling the same volume around the molecules with about 1500 points per
 atom (PDQC method).  The extended high density grid seemed to be
 pretty good and was a bit faster.
 As far as dipole moments are concerned, with ab initio ESP-charges,
 excellent agreement between the SCF and ESP-charge-derived moments can
 be expected (differences of less than 0.01 Debye).  We could not get anywhere
 near as good agreement from the MNDO wavefunction.  The unscaled ESP-charges
 underestimated the MNDO SCF-dipole moment by about 0.3-0.4 Debye.  The scaled
 ESP-charges underestimated the HF/6-31G* SCF-dipole by about 0.1 Debye.
 This feature was reported quite early (J. Comput. Chem. (1990), 11, 431) but
 has not been investigated fully.
 Hope this helps,
 Rob Woods
 Robert J. Woods
 Glycobiology Institute
 Department of Biochemistry
 University of Oxford
 South Parks Road
 Oxford, OX1 3QU