[molpro-user] CBS extrapolations

[Grant Hill]

Firstly, I'd like to prefix this with a statement that CBS extrapolations and the right way to perform them is something of a contentious issue.

Second, I've never used Molpro's default procedure, I much prefer to extract the individual energies and work on them in a spreadsheet (where I have several macros for different procedures). This is not to discourage you from using the built-in one.

Third, I've not inspected the paper you referred to carefully.

1. Unless the basis sets are large I would usually extrapolate the HF energy too. One of the more common ways of doing this it to extrapolate the HF energy with one formula and then use a second formula for the correlation energy. However, there's evidence in the literature that simply extrapolating the total energy with an l_max^-3 based formula will give a result of comparable quality. The best advice I can give here is to read a few of the papers and decide for yourself. J. Chem. Phys. 126, 244105 (2007) is a good place to start.

2. In my experience, DZ calculations are not particularly good and I would personally stick to CBS(3,4). But, I'm sure there are some formulas/cases where a three-point extrapolation is better.

If I was comparing with DFT relative energies, I would probably extrapolate the SCF and correlation energy components separately with two-points. The "spread" from different extrapolation formulas is probably smaller than your target accuracy.

HTH,

Grant

On 22 Sep 2010, at 14:49, John Keith wrote:

> 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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