[molpro-user] CBS extrapolations

[John Keith]

Might someone be able to share expertise doing CBS extrapolations on molecules?

I'd like to benchmark DFT energies of some inorganic complexes against
CCSD(T) methods extrapolated to the CBS limit.  Basically, I'd like to
do a similar study as was done recently in Inorg. Chem. (2010) 5546.
Two questions for someone new at this:

1. Molpro's default procedure appears to extrapolate only the
correlation energy and then add it to the HF energy of the largest
basis set used in the extrapolation.  When in doubt I normally do the
default procedure, but I've seen other papers that give reasons why
extrapolating the total energy, particularly when using Dunning-type
cc-basis sets is preferred.  Does anyone reading this board have
strong feelings one way or the other?

2. One example in the molpro manual shows an extrapolation using only
ATZ and AQZ basis sets.  Naturally, a CBS extrapolation using only two
data points is less ideal than one using 5 or 6, but the size of my
molecules rules out basis sets larger than AQZ.  Would a CBS(2-4)
calculation be more meaningful than a CBS(3-4) calculation?  The
CBS(3-4) calculation is ~0.002 Eh (1.3 kcal/mol) lower than that
CBS(2-4) calculation. Should I trust either of these approximations,
or am I better off just benchmarking to a more definite CCSD(T)/AQZ
value?

Any advice on the matter would be greatly appreciated.

Thanks in advance,

--john

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Ph D. John A. Keith
Institut für Elektrochemie     Email: john.keith at uni-ulm.de
Universität Ulm                    Telephone: +49-731-50-26061
Albert-Einstein-Allee 47       URL: http://www.wag.caltech.edu/home/johnk
Ulm D-89069                       URL:
http://www.uni-ulm.de/theo_echem/index.shtml
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In order to be an immaculate member of a flock of sheep, one must above all,
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