Subject: Re: CCL:G:summary AM1 vs PM3

From: csonka@web
Date: Fri, 3 Nov 1995 10:38:37 +0100

Dear Pjotr,

I was missing the original question and saw only the summary.
I would like to draw your attantion to a very fundamental problem with the core
repulsion function in PM3. It might have an unphysical minima which stabilize
unrealistic H-H, C-H or O-H  interactions.
Some of the consequences were  shown in the paper: J. Comp. Chem. 14 (1993) 895
by me and it was also reported in  QCPE Bull. 11 (1991) 5.
Be careful, because the QCPE figures contain errors.

The reprints of J. Comp. Chem. 14 (1993) 895 are available in limited number
>from me. Meantime I have discovered lot of unpublished problems which are
related with this CRF problem. I have to say the problem is very serious.
If you have structures with H-H distances below 2 angstrom it will occur.

The AM1 is free from this error.

Gabor

-- 

Gabor I. Csonka                 Budapest University of Technology
FAX: (361) 463.36.42            Inorganic Chemistry Dept. Ch. Bldg
[email-masked]           H-1111, Bp. Szent Gellert ter 4
http://www.fsz.bme.hu/bme/chemical/csonka.html

---

From: "Wayne Huang" <[email-masked]>
Date: Fri, 3 Nov 1995 10:39:19 -0800

To echo Gabor Csonka's comments on PM3's problem of unrealistic estimation
of H-H interaction, here are some additional comments on the problem and
fix-up solution.

Original PM3 Bug:
----------------

In original PM3 model, the non-hydrogen interaction are known to be
problematic which is too attractive and gives rise to unusual stability
for crowded arrangements.

							PM3	Expt
							===	===

methyl cyclohexane:  (axial) <-> (equatorial)		1.1	1.8

t-butyl cyclohexane: (axial) <-> (equatorial)		1.1	5.4

Modification:
------------

This can be corrected by introducing a repulsive term between non-bonded
hydrogens into the PM3 energy expression, which allows it rapidly approaches
to zero with increasing H--H distance and becomes insignificant for
distances greater than about 2 angstrom.

						PM3	PM3	Expt
					             (modified)
						===	===	===

methyl cyclohexane:  (axial) <-> (equatorial)	1.1	1.3	1.8

t-butyl cyclohexane: (axial) <-> (equatorial)	1.1	4.8	5.4

This result is little if any change for systems which are not crowded,
but significant change for molecules in which hydorgens are crowded t
together.

This modification is within Spartan semiempirical module. The correction
has been introduced as the default for PM3(tm) (the PM3 method dealing
with d-transition metals) and as an option for PM3.

--Wayne

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