Accuracy assessment of the ROHF-CCSD(T) calculations of static dipole polarizabilities of diatomic radicals:: O2, CN, and NO

Static dipole polarizabilities of three radicals, O-2, CN, and NO were calculated by the coupled cluster CCSD(T) method, with the single determinant restricted open shell Hartree-Fock (ROHF) reference using the finite field method. O-2 and CN are generally considered as difficult systems sensitive to spin contamination. The effect of spin adaptation in the CCSD(T) method with the ROHF reference was considered. Largest single and double excitation amplitudes and the T-1 diagnostics were used as indicators in the quality assessment of calculated dipole polarizabilities. We also discuss these measures in relation to similar quantities for selected closed shell systems for which benchmark results are available. The size and quality of the basis set were systematically improved. Theoretical dipole polarizabilities were corrected for vibration effects and compared with available experimental values. (C) 2001 Elsevier Science B.V. All rights reserved.