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III.F.4. (XVIII.D.1.)

Negative orbital occupation numbers

A negative orbital occupation number indicates that the electronic ground state may not be well represented by a single electronic configuration.

Results for C2H (Ethynyl radical)

The following table lists the minimum orbital occupation numbers


Methods with standard basis sets
3-21G 6-31G* cc-pVTZ aug-cc-pVTZ
Moller Plesset perturbation MP2FC -0.05996 -0.06886 -0.07159 -0.07278
MP2FU   -0.06839    
Coupled Cluster CCD   -0.03016    

The following table lists the T1 diagnostic


Methods with standard basis sets
6-31G*
Coupled Cluster CCSD 0.06842