Subject: compound methods - G2 theory

From: Jeremy R Greenwood <[email-masked]>
Date: Tue, 27 Aug 1996 16:57:50 +1000 (EST)

Greetings all,

I'm attempting to calculate high accuracy energies for a series of species,
and would like to use a compound method.

G2 energies, or failing that G2(MP2) would be the obvious method of choice. 
Unfortunately, G2 is just out of reach; QCISD(T)/6-311G(d,p) is too
expensive, as is MP4. However, QCISD(T)/6-31G(d,p) is not, nor are MP2
calculations.

Questions:

Are there somewhat less expensive compound methods which are recommended?

Would substituting QCISD(T)/6-31G(d,p) for QCISD(T)/6-311G(d,p) make a 
complete nonsense of G2(MP2)? Could the basis-set mismatch be corrected?

Is delta-E-HLC specific to the G-theory basis sets and therefore not portable
to a modified method?

I'm considering the following simple alternative: at MP2/6-31g(d,p) geom
E = (HF)ZPE*0.9 + MP2/6-311G++(3df,3pd) + QCISD(T)/6-31G(d,p) - MP2/6-31g(d,p)

Thanks,

Jeremy

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From: [email-masked] (AEleen Frisch)
Date: Tue, 27 Aug 1996 18:35:33 -0400 (EDT)

> expensive, as is MP4. However, QCISD(T)/6-31G(d,p) is not, nor are MP2

The Complete Basis Set methods (CBS) of Petersson and coworkers are an
attractive alternative, especially CBS-4 and CBS-Q. They are also
compound models. They are automated in G94 and we discuss them
in chapter 7 of Exploring Chemistry.