[molpro-user] why diffuse functions are such efficient

[Kirk Peterson]

It's a little difficult to tell without knowing what A and B actually  
are, but I
would guess that in the avqz multi calculation you might have  
converged to the
wrong state in the separated A+B part or likewise you converged to  
the wrong
state in the vqz calculation for the molecule. I'd check the orbitals  
carefully.

Again without knowing what AB is it is difficult to say for sure, but  
I would say in
most cases such an excellent dissociation energy with MRCI in just a  
vdz basis
set must be fortuitous (or the experimental value is inaccurate).

just my 2 cents,

-Kirk

On Jun 15, 2006, at 8:05 PM, MHT wrote:

>
> Dear Molpro User,
>
> I am calculating D_0, here D_e(A-B)=[E_(A) + E_(B)] - E_(AB).
> Both A and B are doublet radical, AB is a stable molecular.
> active space in multi is full valence.
> By, vqz at multi, the D_e(A-B)=79.77kcal/mol,D_o(A-B)=72.48kcal/mol;
> By, avqz at multi, the D_e(A-B)=106.54kcal/mol,D_o(A-B)=98.332kcal/ 
> mol;
> However, mrci/vdz give D_e(A-B)=93.12kcal/mol,D_o(A-B)=85.83kcal/mol,
> using ZPE from multi/vqz.
>
> the exp value D_o(A-B)=86kcal/mol.
>
> puzzle:
> 1.Why there is so much different between vqz and avqz results?
> 2.Why mrci gives such good result? coincidence?!
>
> thank you and wish all have a very nice weekend!
> MHT
>
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