Theoretical study of the unusual potential energy curve of the A 1Σ+ state of AgH

The ${\mathrm{A }}^1{\Sigma }^{+}$ potential energy curve of AgH is studied by means of the second-order multistate multireference perturbation theory including the spin–orbit and relativistic effects. The anomalous behavior of the vibrational energy levels observed in experiment is reproduced well by theory. An analysis of the ${\mathrm{A }}^1{\Sigma }^{+}$ wave function shows that at the internuclear distance of 4–6 Å, a partial electron transfer from the ${\text{5p}}_z$ orbital of silver to the $\text{1s}$ orbital of hydrogen occurs. This admixture of the ionic-like [core] ${\text{4d}}^{10}{\text{1s}}_H^2$ configuration occurring in ${\mathrm{A }}^1{\Sigma }^{+}$ due to two avoided crossings, namely ${\mathrm{X }}^1{\Sigma }^{+}$ with ${\mathrm{A }}^1{\Sigma }^{+}$ and ${\mathrm{A }}^1{\Sigma }^{+}$ with ${\mathrm{C }}^1{\Sigma }^{+},$ is found to be responsible for the unusual shape of the ${\mathrm{A }}^1{\Sigma }^{+}$ potential energy curve: the effective potential is a superposition of a Morse-like covalent interaction between Ag and H, and the electrostatic ionic-like interaction between ${\mathrm{Ag}}^{+}$ and ${\mathrm{H}}^{-}.$ We present spectroscopic parameters, vibrational levels, and rotational constants computed for a large number of vibrational levels and observe good agreement with available experimental data. The equilibrium distance agrees within 0.01 Å and the vibrational frequency within 60 cm⁻¹ for the state-specific calculations. Larger relative discrepancy is observed for ${\omega }_e{x}_e,$ about 30 cm⁻¹, however the non-Morse-like nature of the energy curve makes it impossible to describe the levels only with ω and ${\omega }_e{x}_e,$ so that direct comparison is not well defined.