I am doing on the CBS extrapolation from the work of Helgaker et al (T. Helgaker, W. Klopper, H. Koch, J. Noga, The Journal of Chemical Physics 106 (1997) 9639). I got into some troubles with the "alpha" parameter of Hartree - Fock energies using by Feller (D. Feller, The Journal of Chemical Physics 96 (1992) 6104.), e.g. E_HF(X) = E_HF(CBS) + B exp(-alpha.X). I cannot find its value in his research, how could I obtain it if I only use two basis set for extrapolating CBS limit. I really appreciate if you give me any comments/advice!
Best wishes,
You will need to find a third energy if you want to fit your extrapolation to this formula; the advice is, also, to stay away from double-zeta basis sets.
However, the Helgaker formula is often also generalized to:
E(X) = E(CBS) + BX-3
and having only two parameters, this latter formula only requires (a minimum of) two energies. However, if you can get more, I'd always suggest doing this; at the same time, I appreciate this is not always possible.
Apologies, I should also have mentioned that you might also want to look at some of the work of Pavel Hobza, who has also done a fair amount of work into CBS extrapolation.
I agree with Mark about using three energies. It is better to solve for three unknowns rather than to fit, I think. With apologies for advertising my own work, please see Eq. (1) of J. Chem. Phys. 119, 9590-9600 (2003).
They all are excellent answers. Here is my experience: three equations need to be solved for three variables. A good set of three basis sets is aug-cc-pVTZ, aug-cc-pVQZ, and aug-cc-pV5Z.