On the Mg(3P)–He(1S) interaction using SA-MCSCF/ICF-CI wave functions

State averaged-multiconfiguration self-consistent-field (SA-MCSCF)/interacting correlated fragment-configuration interaction (ICF-CI) wave functions developed from an extended basis of Slater type orbitals are used to determine the portion of the nonrelativistic Born–Oppenheimer 3Σ+ and 3Π potential energy curves (PEC’s), relevant to fine structure changing collisions in the system Mg(3P)-He(1S). Using the finite perturbation method the parallel and perpendicular polarizabilities of Mg(3P) at the SA-MCSCF level are found to be α∥ =120 a.u.3 and α⊥ =84.5 a.u.3 The PEC’s exhibit the requisite (weak) long-range attraction, with E(3Σ+) α⊥. The 3Σ+ and 3Π curves, which are rigorously degenerate at R(Mg–He)=∞, cross near R(Mg–He)=14 a.u. The 3Σ+ curve exhibits a very shallow minimum (∼1.5 cm−1) near R(Mg–He)=13.7 a.u. while a deeper minimum ∼16 cm−1 is found for the 3Π curve near R(Mg–He)=7.9 a.u. The effects of spin-orbit coupling are incorporated in a semiempirical manner. The SA-MCSCF/ICF-CI approach is compared with the results of a more standard MCSCF/ICF-CI treatment.