Ab initio study of ground state MH2, HMHe+ and MHe22+, M = Mg, Ca

Abstract

The ground states of MH₂, HMHe⁺ and MHe₂²⁺ (M = Mg, Ca) have been investigated using relativistically-corrected CCSD(T), IC-MRCI and IC-MRCI+Q, in conjunction with ANO-RCC (Mg, Ca) and aug-cc-pVQZ (H, He) basis sets. The ground states of all magnesium species are predicted to be linear, in agreement with predicted trends. Conversely, HCaHe⁺ and CaHe₂²⁺ were determined to be quasi-linear species, with linear-inversion barriers of ca. 115 and 3 cm⁻¹, respectively. For CaH₂, a stationary point on the molecular potential energy surface corresponding to a non-linear equilibrium structure was not observed. Trends in bonding, dissociative potential well-depths and spectroscopic constants for these species have been considered with regards to isoelectronic and isovalent reasoning. These trends are consistent with helium and hydrogen forming electrostatic and covalent bonds with the metal ion, respectively.