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.)

Diagnostics on calculations

Species with negative natural orbital occupation numbers

This diagnostic indicates that a given molecule's electronic ground state may not be well represented by a single electronic configuration.
See: "A natural orbital diagnostic for multiconfigurational character in correlated wave functions" M. S. Gordon et al., J. Chem. Phys. 110(9), 4199, 1999
Click on "neg" for model chemistries and occupation eigenvalues.

With larger basis sets (such as aug-cc-pVTZ) many molecules have small negative occupation numbers. To provide a more useful diagnostic the following list is limited to orbital occupation numbers less than -0.001

Species	Name	Minimum Occupation Number
LiH	Lithium Hydride	neg
CH2-	methylene anion	neg
CH3-	methyl anion	neg
NH-	Imidogen anion	neg
NH2-	amino anion	neg
OH-	hydroxide anion	neg
H2O-	water anion	neg
NaH	sodium hydride	neg
HCl-	hydrogen chloride anion	neg
Cu	Copper atom	neg
Li2-	lithium diatomic anion	neg
B2	Boron diatomic	neg
BC-	boron monocarbide anion	neg
BC+	boron monocarbide cation	neg
C2	Carbon diatomic	neg
C2+	carbon diatomic cation	neg
C2H	Ethynyl radical	neg
C2H3	vinyl	neg
BeN	Beryllium mononitride	neg
BeN-	Beryllium mononitride anion	neg
BN-	boron nitride anion	neg
BN	boron nitride	neg
BN+	boron nitride cation	neg
CN	Cyano radical	neg
CN-	cyanide anion	neg
CN+	Cyano cation	neg
HCN+	hydrogen cyanide cation	neg
N2-	nitrogen diatomic anion	neg
NNH	Dinitrogen monohydride	neg
LiO-	lithium oxide anion	neg
LiOH	lithium hydroxide	neg
BeO	beryllium oxide	neg
BeO-	Beryllium monoxide anion	neg
BO	boron monoxide	neg
BO+	boron monoxide cation	neg
CO+	carbon monoxide cation	neg
HCO	Formyl radical	neg
NO	Nitric oxide	neg
NO-	nitric oxide anion	neg
HNO+	Nitrosyl hydride cation	neg
O2-	oxygen diatomic anion	neg
HO2-	Hydroperoxy anion	neg
HO2+	Hydroperoxy cation	neg
LiF	lithium fluoride	neg
BF-	Boron monofluoride anion	neg
FO-	flourine oxide anion	neg
F2-	flourine diatomic anion	neg
NaLi-	lithium sodium anion	neg
Na2-	sodium diatomic anion	neg
NaO	sodium monoxide	neg
NaO-	sodium oxide anion	neg
NaOH	sodium hydroxide	neg
NaF	sodium fluoride	neg
Mg2-	magnesium diatomic anion	neg
MgO	magnesium oxide	neg
MgO-	magnesium oxide anion	neg
MgOH	magnesium hydroxide	neg
MgF+	magnesium fluoride cation	neg
AlN-	aluminum mononitride anion	neg
AlO	Aluminum monoxide	neg
AlO-	Aluminum monoxide anion	neg
AlO+	aluminum monoxide cation	neg
SiC-	silicon monocarbide anion	neg
SiC+	silicon monocarbide cation	neg
SiN	Silicon nitride	neg
SiN-	silicon mononitride anion	neg
SiN+	silicon mononitride cation	neg
SiO-	silicon monoxide anion	neg
SiO+	silicon monoxide cation	neg
CP	Carbon monophosphide	neg
CP-	carbon monophosphide anion	neg
CP+	carbon monophosphide cation	neg
PN-	phosphorus nitride anion	neg
PN+	phosphorus nitride cation	neg
P2-	phosphorus diatomic anion	neg
PO	Phosphorus monoxide	neg
BeS	beryllium sulfide	neg
BS	boron sulfide	neg
BS-	boron monosulfide anion	neg
BS+	boron monosulfide cation	neg
CS-	carbon monosulfide anion	neg
CS+	carbon monosulfide cation	neg
HCS	Thioformyl radical	neg
NS	Mononitrogen monosulfide	neg
SO	Sulfur monoxide	neg
SO-	sulfur monoxide anion	neg
MgS	magnesium sulfide	neg
AlS	Aluminum sulfide	neg
SiS-	silicon monosulfide anion	neg
PS	phosphorus sulfide	neg
BCl-	boron monochloride anion	neg
NCl-	nitrogen monochloride anion	neg
NCl+	nitrogen monochloride cation	neg
ClO	Monochlorine monoxide	neg
ClO-	chlorine monoxide anion	neg
ClF-	clorine monofluoride anion	neg
SCl	sulfur monochloride	neg
CaO	Calcium monoxide	neg
CuO	Copper Monoxide	neg
CuF	Copper monofluoride	neg
CuCl	Copper monochloride	neg
CSe-	Carbon monoselenide anion	neg
SeO	Selenium monoxide	neg
SeO-	selenium monoxide anion	neg
BrF-	bromine fluoride anion	neg
C3H3	Propargyl radical	neg
C3H5	Allyl radical	neg
HN3	hydrogen azide	neg
Li2O	dilithium oxide	neg
HCCO	ketenyl radical	neg
BO2	Boron dioxide	neg
N2O+	Nitrous oxide cation	neg
CO2+	Carbon dioxide cation	neg
CH3CO	Acetyl radical	neg
NO2-	Nitrogen dioxide anion	neg
O3	Ozone	neg
FOO	Dioxygen monofluoride radical	neg
NF2+	Difluoroamino cation	neg
SiO2	silicon dioxide	neg
PO2	Phosphorus dioxide	neg
SO2	Sulfur dioxide	neg
CS2+	Carbon disulfide cation	neg
SSO	Disulfur monoxide	neg
ClOO	chloroperoxy radical	neg
OClO	Chlorine dioxide	neg
SeO2	Selenium dioxide	neg
C4	Carbon tetramer	neg
AlF3	Aluminum trifluoride	neg
SOCl2	thionyl chloride	neg
ClFO3	Perchloryl fluoride	neg
SO2Cl2	Sulfuryl chloride	neg
C6H5	phenyl	neg
C3H4O2S	2H-Thiete-1,1-dioxide	neg
C6H5CH2	benzyl radical	neg