Ground and excited electronic structure analysis of XM4 (X = N, P and M = Li, Na) and their anions

Abstract

High-level coupled-cluster, electron propagator, and multi-reference ab initio methods are employed to study the ground and excited electronic states of the XM₄ (X = N, P and M = Li, Na) series. All XM₄ species bear lower ionization potentials and can be classified as superalkalis. In the ground state each possesses a diffuse electron in the periphery. This expanded electron cloud of tetrahedral NLi₄, NNa₄, and PNa₄ molecules is spherical (similar to an s-orbital) and evenly distributed around the XM₄⁺ core. The outer electron is promoted to higher-angular momentum p-, d-, 2s-type orbitals in excited states. Singly occupied molecular orbitals of excited PLi₄ are deformed due to its lower C₁ symmetry. The aug-cc-pVQZ basis set was found to describe the excited states of XM₄ accurately and efficiently. The bound singlet and triplet electronic states of XM₄⁻ that possess two peripheral electrons are also analyzed.