The dipole moment and magnetic hyperfine properties of the excited A 2Σ+(3sσ) Rydberg state of nitric oxide

Abstract

The dipole moment and magnetic hyperfine properties of the A 2Σ+ Rydberg state of nitric oxide have been evaluated at a variety of levels of theory with extended correlation consistent basis sets. Using the finite field approach to compute the dipole moment, restricted coupled cluster RCCSD(T) and complete active space-configuration interaction CAS-CI+Q methods yield values (1.09–1.12 D) that are essentially identical to experiment. In contrast, dipole moments computed as an expectation value of the dipole moment operator typically differ from experiment by 0.1–0.6 D. The rather unfavorable comparisons with experiment reported in previous theoretical studies may stem, in part, from the method chosen to evaluate the dipole moment. Magnetic hyperfine properties were evaluated using a variety of unrestricted and restricted open-shell Hartree–Fock-based methods. We estimated the full CI limiting properties by exploiting the convergence behavior of a sequence of MRCI wave functions. The isotropic component Aiso(14N) of 39±1 MHz evaluated in this fashion is in excellent accord with the experimental value of 41.4±1.7 MHz. Highly correlated UHF-based methods [e.g., CCSD(T) and QCISD(T)] yield comparable values of 40–41 MHz that are in good agreement with both experiment and the apparent full CI limit. However, for Aiso(17O), the full CI limit (−97±2 MHz) and the UHF-based results (ca.−118 MHz) differ by roughly 20 MHz. It remains unclear how to reconcile this large discrepancy.