A correlated ab initio study of the Ã2Π ← X̃2Σ+ transition in MgCCH

The Ã2Π ← X̃2Σ+ transition in MgCCH was studied with correlation consistent basis sets and single- and multireference correlation methods. The Ã2Π excited state was characterized in detail; the X̃2Σ+ ground state has been described elsewhere recently. The estimated complete basis set (CBS) limits for valence correlation, including zero-point energy corrections, are 22668, 23191, and 22795 cm−1 for the RCCSD(T), MRCI, and MRCI + Q methods, respectively. A core-valence correction of +162 cm−1 shifts the RCCSD(T) value to 22830 cm−1, in good agreement with the experimental result of 22807 cm−1.