The electronic states of Al₃

The electronic and geometrical structural properties of Al⁺₃, Al₃ and Al^-₃ have been investigated using an ab initio complete active space, multi-configuration self-consistent field method in a double-zeta plus polarization Gaussian basis set. The ten core electrons of each Al atom were replaced by an accurate compact effective core potential. Al⁺₃ is predicted to be linear and Al^-₃ is calculated to have near-D_3h, symmetry. The neutral trimer has a near-equilateral triangle static geometry. The electronic ground states of Al⁺₃ and Al^-₃ are both predicted to be relatively unreactive ¹A₁ closed-shell species. Al₃ is calculated to hav state, in disagreement with recent interpretations of ESR spectra in matrix isolation as favoring a spin-quartet ground state.