Ab initio calculations of the parallel component of the magnetic dipole hyperfine structure (HFS) constant have been carried out for hydroxyl radical isotopologues [${}^{16,17}\mathrm{OH}$(D)] over the internuclear distance range $R\in [0.6,1.8]\AA$. For the ground electronic state $X^2\mathrm{\Pi }$, 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 $A^2{\mathrm{\Sigma }}^{+}$ state. The quantum-chemistry study employs a four-component relativistic coupled-cluster method, including excitations up to the triple level, namely: the contribution of triple-cluster amplitudes was studied both perturbatively and through fully iterative calculations. 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 $X^2\mathrm{\Pi }$ state. Vibrationally averaged 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 nonadiabatic frameworks.

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• Atomic & molecular structure
• Electronic structure of atoms & molecules
• Fine & hyperfine structure
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions