A relativistic coupled-cluster treatment of magnetic hyperfine structure of the $X^2\Pi$ and $A^2\Sigma^+$ states of OH isotopologues

Abstract

$\textit{Ab initio}$ calculations of the parallel component of the magnetic dipole hyperfine structure (HFS) constant have been carried out for hydroxyl radical isotopologues ($^{16,17}$OH(D)) over the internuclear distance range $R \in [0.6, 1.8]$ \r{A}. For the ground electronic state $X^2\Pi$, the HFS functions were evaluated for contributions induced by both oxygen and hydrogen nuclei. In addition, the hydrogen-induced HFS curve was calculated for the excited $A^2\Sigma^+$ state. The quantum-chemistry study employs a four-component relativistic coupled-cluster (CC) method, including excitations up to the triple level, namely: the contribution of triple-cluster amplitudes was studied both perturbatively (CCSD(T)) and through fully iterative calculations (CCSDT). The resulting oxygen- and hydrogen-induced HFS functions represent the most accurate and reliable theoretical predictions to date exhibiting excellent agreement with semiempirical curve for hydrogen-induced HFS derived from high-resolution spectroscopic data for the lowest vibrational levels ($v\in [0,2]$) of the electronic $X^2\Pi$ state. Vibrationally averaged $\textit{ab initio}$ values are consistent with experimental values within $1\%$ for all states considered. Furthermore, the internuclear distance range over which the HFS curves are defined has been extended beyond that of previous studies, thereby providing a robust foundation for accurate HFS treatments of higher-lying rovibrational levels of OH isotopologues within both adiabatic and non-adiabatic frameworks.