Spectroscopic Properties of MgH₂, MgD₂, and MgHD Calculated from a New ab Initio Potential Energy Surface

A three-dimensional potential energy surface for the ground electronic state of MgH₂ has been constructed from 9030 symmetry-unique ab initio points calculated using the icMRCI+Q method with aug-cc-pVnZ basis sets for n = 3, 4, and 5, with core-electron correlation calculated at the MR-ACPF level of theory using cc-pCVnZ basis sets, with both calculations being extrapolated to the complete basis set limit. Calculated spectroscopic constants of MgH₂ and MgD₂ are in excellent agreement with recent experimental results:  for four bands of MgH₂ and one band of MgD₂ the root-mean-square (rms) band origin discrepancies were only 0.44 and 0.06 cm^-1, respectively, and the rms relative discrepancies in the inertial rotational constants (B_[v]) were only 0.0196% and 0.0058%, respectively. Spectroscopic constants for MgHD were predicted using the same potential surface.