[molpro-user] why diffuse functions are such efficient

[MHT]

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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