\ CCCBDB Diagnostics on calculations
National Institute of Standards and Technology
Computational Chemistry Comparison and Benchmark DataBase
Release 21August 2020
NIST Standard Reference Database 101
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IIExperimental data
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XIXIndex of properties
XXH-bond dimers
XXIOddities

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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 C2H3 (vinyl)

The following table lists the minimum orbital occupation numbers


Methods with standard basis sets
3-21G 6-31G* cc-pVTZ aug-cc-pVTZ
3-21G 6-31G* cc-pVTZ aug-cc-pVTZ
Moller Plesset perturbation MP2 -0.06813 -0.08845 -0.09854 -0.10100
MP2=FULL   -0.08803    
Coupled Cluster CCD   -0.04733    
3-21G 6-31G* cc-pVTZ aug-cc-pVTZ

The following table lists the T1 diagnostic


Methods with standard basis sets
6-31G*
Coupled Cluster CCSD 0.04743
6-31G*