Characterization of the three lowest-lying singlet electronic states of AlOH

Two linear (1Σ+ and 1Π) and three bent (1 1A′, 2 1A′, and 1 1A″) lowest-lying electronic singlet states of AlOH have been systematically investigated employing ab initio self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and CCSD with iterative partial triple excitations (CCSDT-3 and CC3) quantum mechanical methods with basis sets up to augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ). The linear 1Σ+ state is found to be a remarkably low-energy transition state between the two equivalent bent 1 1A′ structures, while the linear 1Π state is a second-order saddle point, which leads to the bent 2 1A′ and 1 1A″ states. The bent ground (X 1A′) state of AlOH is predicted to have a bond angle of 157° at the aug-cc-pVQZ CC3 level of theory and is classified as a quasilinear molecule, confirming previous studies. Employing the equation-of-m...