\ CCCBDB Diagnostics on calculations
National Institute of Standards and Technology
Computational Chemistry Comparison and Benchmark DataBase
Release 22May 2022
NIST Standard Reference Database 101
IIntroduction
IIExperimental data
IIICalculated data
IVData comparisons
VCost comparisons
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XVEntropy 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 MgS (magnesium sulfide)

The following table lists the minimum orbital occupation numbers


Methods with standard basis sets
3-21G 6-31G* aug-cc-pVTZ
3-21G 6-31G* aug-cc-pVTZ
Moller Plesset perturbation MP2 -0.00190 -0.00160 -0.00057
MP2=FULL   -0.00140  
Coupled Cluster CCD   -0.00015  
3-21G 6-31G* aug-cc-pVTZ

The following table lists the T1 diagnostic


Methods with standard basis sets
6-31G*
Coupled Cluster CCSD 0.03987
6-31G*