Re: CCL:summary AM1 vs PM3

 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
 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
 						===	===	===
 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
 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.
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