Sign/criterium of possible multiconfigurational character of the ground state

[evgeniy]

Dear Users and Developers of Turbomole,

I wonder what is the criterium of single reference ground state WF in a CC2 calculation.
In my case I have D1 diagnostic of 0.1372, which is somewhat larger than it normally should
be in the case of single reference ground state. Would calculation of CC2 natural occupations
and the D2 diagnostic help to clarify it?

Best regards,
Evgeniy

[christof.haettig]

The D2 diagnostic gives an information which is complementary to the D1 diagnostic.

D1 is only a diagnostic for configurations reached by single excitations out of the reference determinant,
D2 is a diagnostic for configurations reached by double excitations out of the reference determinant.

The natural occupation numbers include for CC2 information about both.

Christof