Subject: fast esp charges from AM1 calculations

From: "George P. Ford" <[email-masked]>
Date: Fri, 7 Jun 96 12:28 CDT

Dear netters:

Alex Turner (University of Bath) recently posted the details of an AM1 esp 
calculation on a singly hydrated chloromethoxymethane using mopac93. The input 
file is reproduced below

AM1  esp

C    0.00000000  0    0.0000000  0    0.0000000  0    0    0    0
H    1.11678955  1    0.0000000  0    0.0000000  0    1    0    0
H    1.11860861  1  110.5001057  1    0.0000000  0    1    2    0
H    1.11804978  1  111.0078215  1  124.1366815  1    1    2    3
O    1.42977966  1  109.6919909  1 -121.1538376  1    1    2    3
C    1.39974700  1  114.4456389  1   68.8811837  1    5    1    2
H    1.11840798  1  106.4911216  1  161.8104747  1    6    5    1
H    1.11884361  1  112.9830933  1   35.3301012  1    6    5    1
Cl   1.80283055  1  111.4239110  1  -83.0814824  1    6    5    1
O    3.42808614  1   97.1266757  1   20.8564401  1    6    5    1
H    0.96400805  1  126.7099492  1   68.1397632  1   10    6    5
H    0.96345463  1  101.2179497  1  -48.2960684  1   10    6    5

Alex wrote:

>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.
>
>It has been brought to my attention that Mopac93 _revision_2_ should
>have a bug fix (Dr J Stuart - thanks), But I have not had time to
>check this out yet.
>
Given the arbitrary nature of any definition of atomic charge differences are 
bound to occur. However, from the out file Alex posted it's obvious his 
calculation has a much more serious problem. The dipole moment given by the esp 
charge distribution is a ridiculous 37.2 D!

I donÕt know whether this is a bug in mopac or not. (Any body have any info on 
this?) However, quite apart from this possibility, there is an important reason 
to be distrustful of the standard approach to the calculation of esp's based on 
AM1 and related wavefunctions (deorthogonalized or not). We have made the point 
before (B.Wang; G.P. Ford, J. Comput. Chem, 1994, 15, 200-207), but itÕs worth 
reiterating here. The standard approach is inherently inconsistent since the 
electronic contibution to the esp is computed using a wave function based on 
semiempirical values of the integrals, while the nuclear contribution is 
appropriate to an exact calculation. This problem doesnÕt arise in an AM1 energy
calculation because the necessary balance is restored through parameterization 
of the core repulsion function.

We have used the same philosophy to restore the balance in AM1 esp calculations 
(see JCC paper cited above) by parametrizing the nuclear contribution to 
reproduce the results of HF/6-31G* espÕs. We dubbed this approach the 
Parameterized Molecular Electrostatic Potential (PMEP). The resulting 
PMEP-derived charges are much closer to the ab initio esp charges. Because the 
electronic part is computed strictly within the simplified integral scheme of 
the AM1 formalism the method is EXTREMELY FAST.

The results for AlexÕs molecule are shown below (all refer to the optimized AM1 
geometry). The first set of charges is from Alex's out file. The negatively 
charged carbon he referred to is atom No. 6. 

  The next column shows the AM1 (PMEP) charges and the last shows the HF/6-31G* 
values (g94 with POP=MK). The two latter sets are qualitatively much closer to 
one another than either is to the standard mopac esp calculation. All methods 
still make C6 slightly negative, although the CH2 group as a whole in positive 
in the latter methods which also make the Cl markedly more negative.

ATOM NO.    TYPE    CHARGE
                    (esp)         (qpmep)     HF/6-31G*
  1          C    -0.4073        -0.1113      -0.021124
  2          H     0.1575         0.0952       0.085615
  3          H     0.1508         0.0919       0.055452
  4          H     0.1626         0.1040       0.072576
  5          O     0.0241        -0.2437      -0.312698
  6          C    -0.7417        -0.1549      -0.027673
  7          H     0.3030         0.1783       0.140668
  8          H     0.3827         0.1615       0.205274
  9         Cl    -0.0028        -0.1407      -0.206185
 10          O    -0.5600        -0.5888      -0.841931
 11          H     0.2721         0.3114       0.430359
 12          H     0.2590         0.2970       0.419665

total dipole         37.2          4.04           4.97

The QPMEP calculation required only 0.2 sec. on an SGI Indigo2 compared to the 
217 secs. required for the full HF/6-31G* calculation in g94. The QPMEP 
calculations are available in the most recent release of moapc93 using the QPMEP
keyword. Users of mopac6 can obtain the necessary subroutines via anonymous ftp 
from: ftp.seas.smu.edu.

============================================================================
George P. Ford                      |     email:  [email-masked]
Department of Chemistry             |       web:  http://www.smu.edu/~gford/
Southern Methodist University       | telephone:  (214)768-2479
Dallas, Texas 75275                 |       fax:  (214)768-4089
============================================================================