Full configuration-interaction benchmark calculations for AlH

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Abstract
The X 1Σ+, A 1Π, and C 1Σ+ states of AlH are studied using a [7s 5p 2d/4s 3p] basis set and incorporating valence correlation using the complete active space self‐consistent‐field (CASSCF)/second‐order configuration‐interaction (SOCI) and full configuration‐interaction (FCI) methods. A principal focus of this work is to provide FCI calibration calculations for approximate methods of describing the energies, dipole moments, and transition moments when there is a variable degree of valence–Rydberg mixing. The state‐averaged CASSCF/SOCI method is found to be capable of providing an accurate description of these states when the CASSCF active space is sufficiently flexible to describe both the important correlation effects and the Rydberg character. Having found a CASSCF/SOCI procedure that nearly reproduces the FCI results in a [7s 5p 2d/4s 3p] basis set, we take this procedure to near the basis set limit. The calculated dissociation energies (De in eV with experimental values in parentheses) are 3.16 (3.16) for the X 1Σ+ state, 0.24 (0.24) for the A 1Π state, and 0.80 (0.76) for the inner minimum of the C 1Σ+ state. Our calculated radiative lifetimes for the v=0, N=5 and v=1, N=7 levels of the A 1Π state are 64.3 and 96.6 ns, in quite good agreement with a previous CASSCF study, and with the experimental values of 66±4 and 83±6 ns. The radiative lifetimes for the v=0, N=0 and v=1, N=0 levels of the inner minimum of the C 1Σ+ state are 5.5 and 4.1 ns, respectively.