National Institute of Standards and Technology
Computational Chemistry Comparison and Benchmark DataBase
Release 22May 2022
NIST Standard Reference Database 101
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XVII.C.1. (III.D.6.)

Calculated Ionization Energy for C2 (Carbon diatomic)

Experimental Ionization Energy is 11.4 ± 0.3 eV
Please note! These calculated ionizataion energies have the vibrational zero-point energy (zpe) included, but the zpe has NOT been scaled. Click on an entry for more details, including the ionization energy with a scaled zpe.
Original data displayed. Press to display differences.
Differences displayed. Press to display original data.
Ionization Energies in eV
Methods with predefined basis sets
semi-empirical PM3  
composite G2 12.022
G3 11.912
G3B3 11.904
G3MP2 11.886
G4 11.945
CBS-Q 11.912
Ionization Energies in eV
Methods with standard basis sets
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ daug-cc-pVTZ
hartree fock HF 6.012 8.069 8.069 7.766 7.731 7.724 7.809 7.791 7.791 7.632   7.836 7.757 7.755 7.764 7.709 7.755 7.762 7.753
density functional LSDA 10.418 12.324 12.324 12.058 11.932 11.932 12.059 12.145 12.145 11.889   9.375 11.986 12.137   11.996      
BLYP 9.292 11.184 11.184 10.945 10.824 10.824 10.993 11.042 11.042 10.790   8.894 10.907 11.060          
B1B95 9.189 11.031 11.030 10.791 10.575 10.678 10.787 10.793 10.793 10.628   10.743 10.713 10.690          
B3LYP 9.180 11.072 11.072 10.813 10.698 10.698 10.837 10.877 10.877 10.652   10.924 10.763 10.882 10.895 10.779 10.886 10.896  
B3LYPultrafine   11.072     10.698 10.698 10.837 10.877       10.924 10.763 10.882   10.779 10.886    
B3PW91 9.201 11.084 11.084 10.787 10.681 10.681 10.778 10.807 10.807 10.626   10.848 10.718 10.806          
mPW1PW91 9.072 10.935 10.956 10.658 10.534 10.534 10.637 10.651 10.670 10.497   10.725 10.569 10.657   10.575 10.678    
M06-2X 8.899 10.751 10.751 10.523 10.436 10.436 10.547 10.561 10.561 10.390 10.586 10.595 10.469 10.564   10.465 10.566    
PBEPBE 9.490 11.369 11.369 11.110 10.372 10.990 8.404 8.399 8.399 8.105   11.208 8.411 8.431   11.062 11.177    
PBEPBEultrafine   11.369     8.120 10.990 11.123 11.151       11.208 11.047 11.170   11.062 11.177    
PBE1PBE 9.058 10.959 10.959 10.672 10.568 10.568 10.671 10.686 10.686 10.510   10.738 10.606 10.692   10.592 10.693    
HSEh1PBE 9.046 12.363 10.950 10.664 10.560 10.560 10.558 10.685 10.685 10.503   10.735 10.599 13.174   10.587 10.692    
TPSSh   10.950 10.950 10.696 10.577 10.577 10.679 10.689   10.540   10.747 10.616 10.712   10.607 10.715    
wB97X-D     10.874   10.466   10.560   10.572     10.615 10.560 10.553     10.553    
B97D3   11.106     9.478   9.746   9.682   9.762 9.758   10.865     9.742   10.870
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ daug-cc-pVTZ
Moller Plesset perturbation MP2 9.923 11.919 11.919 11.676 11.653 11.653 11.752 11.849 11.849 11.893   11.860 11.741 12.087 12.228 11.791 12.135 12.254  
MP2=FULL 9.928 11.923 11.923 11.681 11.674 11.674 11.773 11.869 11.869 11.926   11.878 11.756 12.147 12.273 11.807 12.209 12.300  
MP3         10.035   10.035         10.184 10.118 10.432          
MP3=FULL         10.052   10.138         10.196 10.128 10.495          
MP4   11.860     11.659       11.878     11.910 11.721 12.153   11.772 12.193    
MP4=FULL   11.864     11.682       11.900       11.734 12.220   11.786 12.276    
B2PLYP 9.574 11.501 11.501 11.248 11.156 11.156 11.274 11.333 11.333 11.198   11.369 11.229 11.407   11.251 11.425    
B2PLYP=FULL 9.576 11.502 11.502 11.250 11.163 11.163 11.281 11.339 11.339 11.209   11.375 11.233 11.425   11.256 11.447    
B2PLYP=FULLultrafine         11.163               11.233 11.425     11.447    
Configuration interaction CID   10.285 10.285 9.986 9.966     10.090                      
CISD   10.421 10.421 10.120 10.108     10.228                      
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ daug-cc-pVTZ
Quadratic configuration interaction QCISD   11.282 11.282 10.995 10.937 10.937 11.016 11.063 11.063 11.106   11.102 10.986 11.247   11.005 11.276    
QCISD(T)         11.301             11.499 11.360 11.683   11.397 11.719    
QCISD(T)=FULL         11.325   11.409           11.372 11.756 11.858 11.410 11.809 11.874  
QCISD(TQ)         11.170   11.248           11.229 11.534 11.648 11.268 11.571    
QCISD(TQ)=FULL         11.194   11.273           11.242 11.609 11.705 11.282 11.662    
Coupled Cluster CCD   10.947 10.947 10.657 10.592 10.592 10.676 10.720 10.720 10.771   10.753 10.644 10.916   10.670 10.951    
CCSD         10.824         10.994   10.986 10.876 11.132 11.231 10.896 11.162 11.245  
CCSD=FULL         10.842         11.026   10.997 10.886 11.193 11.272 10.906 11.239 11.286  
CCSD(T)         11.277             11.474 11.335 11.652 11.768 11.373 11.689 11.784  
CCSD(T)=FULL         11.301             11.493 11.347 11.724 11.824 11.387 11.778 11.840  
STO-3G 3-21G 3-21G* 6-31G 6-31G* 6-31G** 6-31+G** 6-311G* 6-311G** 6-31G(2df,p) 6-311+G(3df,2p) TZVP cc-pVDZ cc-pVTZ cc-pVQZ aug-cc-pVDZ aug-cc-pVTZ aug-cc-pVQZ daug-cc-pVTZ
Ionization Energies in eV
Methods with effective core potentials (select basis sets)
CEP-31G CEP-31G* CEP-121G CEP-121G* LANL2DZ SDD cc-pVTZ-PP aug-cc-pVTZ-PP Def2TZVPP
hartree fock HF 7.702 7.661 7.732 7.758 7.884 7.877     7.763
density functional B3LYP 10.518 10.434 10.543 10.525 10.932 10.903     10.880
PBEPBE                 9.777
Moller Plesset perturbation MP2 11.314 11.333 11.556 11.614 11.760 11.766     12.108
For descriptions of the methods (AM1, HF, MP2, ...) and basis sets (3-21G, 3-21G*, 6-31G, ...) see the glossary in section I.C. Predefined means the basis set used is determined by the method.