Electronic states and nature of the chemical bond in the molecule CrC by all‐electron ab initio calculations

All‐electron ab initio Hartree–Fock ( HF ), valence configuration interaction ( CI ), and multiconfiguration self‐consistent‐field ( CASSCF ) calculations have been applied to investigate the electronic states of the CrC molecule. The molecule is predicted as having four low‐lying electronic states, 3 ∑ − , 5 ∑ − , 7 ∑ − , and 9 ∑ − , separated by an energy gap of 0.55 eV from the next higher‐lying state, 1 ∑ − , which is followed by the states 5 Π and 7 Π. The four lowest‐lying electronic states are due to the coupling of the angular momenta of the 6 S g Cr + ion with those of the 4 S u C − anion. The chemical bond in the 3 ∑ − ground state can be viewed as a quadruple bond composed of two σ and two π bonds. One σ bond is due to the formation of a molecular orbital that is doubly occupied. The remaining bonds, i.e., one σ and two π bonds, arise from valence‐bond couplings. The π bonds originate from the valence‐bond couplings of the electrons in the C 2 p π orbitals with those in the Cr 3 d π orbitals. The σ bond originates from the valence‐bond coupling of the C 2 p σ electron with an electron in the Cr 4 s , 4 p hybrid that is polarized away from the C atom.