Subject: semiempircal calculations simple oxides

From: <[email-masked]>
Date: 27 Feb 93 14:02:12 EDT

E. M. EVLETH
Dynamique des Interactions Moleculaires
Universite Pierre et Marie Curie
4 Place Jussieu, Tour 22, Paris 75005
33-1-44-27-42-08 (work), 33 = France; 1 = Paris
33-1-45-48-67-20 (home)
FAX 33-1-44-27-42-17
e-mail UDIM018 at FRORS31

The semiemprical parameterizations developed by Dewar's group
never have produced 'good' results on nitrogen and chlorine
oxides, peroxides etc.  and PM3 is no better. STD heats of
formation errors are in the 20 kcal region, structures sometimes
bizare. Even molecular triplet oxygen, which should be zero is
20 kcals off (AM1, MINDO3 is better).    You'll have better 'luck'
sticking to large polyatomics, and organics by preference.
Unfortunately nobody has published a list of 'dos and don'ts'
in semiempirical methods, a poor calculation can serve as
a good example. But those committed to semiempirical methodes
prefer an upbeat approach and speak no evil. Ab initioists dont care
about semiemprical screwups but have their lists of pathological
structures (NO3 radical, ClOO, FOOF etc) too, but can work hard to
getting them right. And you have to be a relatively attentive
doctor to know when your structure is sick. NO2 and NO
monomers and dimers are strange beasts. Beware