National Institute of Standards and Technology

Computational Chemistry Comparison and Benchmark DataBase
Release 22	May 2022
NIST Standard Reference Database 101

I	Introduction
II	Experimental data
III	Calculated data
IV	Data comparisons
V	Cost comparisons
VI	Input and output files
VII	Tutorials and Units
VIII	Links to other sites
IX	Feedback
X	Older CCCBDB versions
XII	Geometries
XIII	Vibrations
XIV	Reaction data
XV	Entropy data
XVI	Bibliographic data
XVII	Ion data
XVIII	Bad calculations
XIX	Index of properties
XX	H-bond dimers
XXI	Oddities

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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 C₂ (Carbon diatomic)

The following table lists the minimum orbital occupation numbers

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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	-0.00420	-0.00207	-0.00154	-0.00160
	MP2=FULL		-0.00206
Coupled Cluster	CCD		-0.00002
		3-21G	6-31G*	cc-pVTZ	aug-cc-pVTZ

The following table lists the T1 diagnostic

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Methods with standard basis sets

		6-31G*
Coupled Cluster	CCSD	0.03858
		6-31G*