MOPAC6.0 ESP, Again
Dear CyberChemists,
Calcn. of ESP fitted charges with MOPAC6.0
_________________________________________
a collegue has been using MOPAC6.0's ESP option to get a set atom
charges fitted (least squares) to a bunch of electrostatic potential
points on Connolly surfaces at 1.4, 1.6,1.8 and 2.0 times the vdWs
atom radius. Old hat, yes, but he is unable to reproduce the values
that appear in Kolman's paper (J. Comp. Chem., Vol. 11, No. 4, 431-
439 (1990)) and neither am I. Consider the case of formaldehyde
(does anyone call it methanal?). I'm using the default values for
SCALE (1.4) NSURF(4) and SCINCR (0.2). Here's my input deck . .
GRAPH NODIIS MMOK ESP SLOPE=1.422 POTWRT NOINTER PRECISE
I shot an elephant in my pyjamas the other day
C 0.000000 0 0.000000 0 0.000000 0 0 0 0
O 1.230000 1 0.000000 0 0.000000 0 1 0 0
H 1.090000 1 120.000092 1 0.000000 0 1 2 0
H 1.070000 1 120.000114 1 180.000000 1 1 2 3
The fitted charges are . . (from .out)
ELECTROSTATIC POTENTIAL CHARGES
ATOM NO. TYPE CHARGE SCALED CHARGE
1 C 0.3897 0.5541
2 O -0.3613 -0.5137
3 H -0.0142 -0.0202
4 H -0.0142 -0.0202
THE NUMBER OF POINTS IS: 375
THE RMS DEVIATION IS: 0.7655
THE RRMS DEVIATION IS: 0.0794
While Kollman et al report (in figure 1) the following MNDO charges.
Formaldehyde
H +0.006
C +0.339
O -0.352
The unscaled fitted charges aren't too bad, but the H's have changed sign.
The scaled fitted charges seem incredible and your humble correspondent
does not believe them. Have I missed something obvious?
Yours faithfully,
Max Muir