Collect. Czech. Chem. Commun. 1998, 63, 1409-1430

Accuracy Assessment of the ROHF-CCSD(T) Calculationsof Dipole Moments of Small Radicals

Miroslav Urbana, Pavel Neográdya, Juraj Raaba and Geerd H. F. Diercksenb

a Department of Physical Chemistry, Faculty of Sciences, Comenius University, Mlynská dolina, SK-842 15 Bratislava, Slovak Republic
b Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, POB 1523, 857 40 Garching, Germany


Dipole moments of a series of radicals, OH, NO, NS, SF, SO, PO, ClO, CN, LiO, NO2, and ClO2 were calculated by the Coupled Cluster CCSD(T) method with the single determinant restricted open shell Hartree-Fock (ROHF) reference. For all molecules theoretical dipole moments were carefully compared to experimental values. The size and the quality of the basis set were systematically improved. Spin adaptation in the ROHF-CCSD(T) method, largest single and double excitation amplitudes and the T1 diagnostics were considered as indicators in the quality assessment of calculated dipole moments. For most molecules the accuracy within 0.01-0.03 D was readily obtained. For ClO and CN the spin adaptation was necessary - its contribution was as large as 0.03-0.045 D. Large deviation from experiment is observed for OH in its A2Σ+ excited state (0.135 D) and especially for LiO in its 2Π ground state (0.22 D). No indication of the failure of theoretical calculations was found which leads to the conclusion that, even if there is still a space for the improvement of theoretical calculations, experimental values should be reconsidered.

Keywords: Radicals; Dipole moments; CCSD(T); ROHF; Spin-adaptation; Ab initio calculations.