Subject: summary: compound methods / G2 theory

From: Jeremy R Greenwood <[email-masked]>
Date: Mon, 2 Sep 1996 19:10:48 +1000 (EST)

Greetings all,

I recently asked a question about cheaper compound methods than G2(MP2)
and was inundated with excellent advice. Thanks to all who replied; a
short trip to the library and I'd found a good solution within a day.

Suggestions were generally of the following nature:

* Try DFT or hybrid DFT methods instead
* Try CBS methods instead
* Try simplifying the problem by only performing high level correlation
  calculations on the most important part of the molecules
* Try the G2(MP2,SVP) method of Smith, Radom, et al.

The solution I've chosen is a modification of the latter; G2 energies
include an empirical, basis-set-specific deltaE-HLC term, which is a linear 
combination of the number of alpha and beta electrons. Since all my systems 
have the same number of these, and since I'm most interested in relative 
energies, this term cancels, and I'm free to use the best basis sets 
I can afford.

G2(MP2) = ZPE*.8929 + HLC(G2) + QCISD(T)/6-311g(d,p) + MP2/6-311+G(3df,2p) 
                    - MP2/6-311g(d,p)
          // MP2/6-31g(d)

G2(MP2,SVP) = ZPE*.8929 + HLC(G2,SVP) + QCISD(T)/6-31g(d) + MP2/6-311+G(3df,2p)
                        - MP2/6-31g(d)
          // MP2/6-31g(d)

my method of choice:

deltaE = delta(ZPE*.8929 + QCISD(T)/6-31g(d,p) + MP2/6-311++G(3df,3pd)
                         - MP2/6-31g(d,p)  )
          // MP2/6-31g(d,p)

Jeremy
******************************************************************************
Original Question:

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)?

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

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
*****************************************************************************
Answers: