Study of isolated 1 : 1 AI+3.He complex using many-body perturbation theory: A multi-reference approach

Abstract

Development and applications of many-body methods capable of reliable computation of potential energy curves (PEC) containing avoided curve-crossing zone remains a challenging area of activity in electronic structure theory till date. In this paper, we present the PEC of the ground state and the corresponding lowest charge transfer state of an isolated 1 : 1 AI+3.He complex, calculated using the third order effective valence shell Hamiltonian, Hv 3rd, method (a variant of quasi-degenerate multi-reference perturbation theory) and then analyze the aspect of avoided curve-crossing between these two states of the complex. The published theoretical data of this complex is very limited and to our knowledge no experimental data has been reported yet for the ground and excited electronic states of this complex. Thus, a comprehensive theoretical investigation of such a complex would be very useful to identify its structural properties. In our numerical analysis, a comparison of our findings is made with the results generated via other many-body methods whenever available. The results obtained from our method appear quite promising, implying the efficacy of Hv 3rd method to compute the energy surfaces (with degeneracy/quasi-degeneracy of varying degrees) over a wide range of geometries including the avoided curve-crossing zone. We further report the ground state spectroscopic constants of this complex obtained from the improved virtual orbital-complete active space configuration interaction (IVO-CASCI) based numerical gradient method. The spectroscopic constants predicted by the IVO-CASCI are in agreement with those determined from the state-of-the-art coupled cluster method.