Energy surfaces of low-lying states of C3

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Abstract
Density functional calculations have been performed for the energy surfaces of the ground state [1Σ+g(1A1)] and low‐lying excited states [3Πu(3B1,3A1), 3Πg(3B2,3A2), 1Πu, 1Πg] of the C3 molecule. The experimental geometries and symmetric stretch frequencies of 1A1 and 1Πu states are reproduced well, and there is good agreement with CI calculations for the triplet states. The energy surface of the ground state is described qualitatively better than by CI calculations and does not show the unusual square‐well form found in Hartree–Fock calculations. The splitting of the Born–Oppenheimer surfaces in the Π states (Renner–Teller effect) is discussed, and the qualitative features of the bonding presented in terms of the geometry dependence of the one‐electron eigenvalues (Walsh diagrams). Together with earlier results for C2, the calculations provide insight into the unique features of the C–C bond.