Subject: ELECTRON CORRELATION AND SEMIEMPIRICAL CALCS

From: "E. Lewars" <[email-masked]>
Date: Wed, 11 Oct 1995 17:24:21 -0400

Re Hank Verhaar's question on whether semiemp heats of form's should get 
better if electron correlation is include, as is supposed to happen with
ab initio.
Of course ab in calcs don't give heats of formation directlty; this info'
must be coaxed out of them in a roundabout way, usually involving appeal to 
experimental quantities somewhere.  Semiemp methods (some of them anyway)
are calibrated against exp to reproduce heats of form'n, so it is not 
clear why specific electron correlation routines need to be included;
indeed, Tim Clark says (A Handbook of Computational Chemistry, 1985,
p 97) "An important point to note is that MINDO/3 and MNDO are parametrized
at the SCF level to fit experimental results, which already reflect correlation.  Therefore any calculations using these methods and considering correlation
do not give realistic heats of formation, because {the correlation energy has
been included twice, once via the parametrization and once explicitly."
  Nevertheless, there does seem to be a place for e.g. CI in semiemp 
methods, e.g. in lowering the energies of transition states to more
realistic values as in Thiele's MNDOC. 
===
Errol Lewars
====