Computational Chemistry Comparison and Benchmark DataBase Release 22 (May 2022) Standard Reference Database 101 National Institute of Standards and Technology
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Geometry for C₄ (Carbon tetramer) ³Σ_g D*H

mPW1PW91/aug-cc-pVDZ

For information on specific bond angles or dihedrals see the geometry comparison page in section Comparisons > Geometry > Bonds, angles.