We present well-parallelized local implementations of high-spin open-shell coupled cluster methods with single and double excitations (CCSD) using pair natural orbitals (PNOs). The methods are based on the spin-orbital coupled cluster theory using restricted open-shell Hartree-Fock (ROHF) reference functions. Two variants, namely, PNO-UCCSD and PNO-RCCSD are implemented and compared. In PNO-UCCSD, the coupled cluster amplitudes are spin-unrestricted, while in PNO-RCCSD the linear terms are spin-adapted by a spin-projection approach as described in J. Chem. Phys. 1993, 99, 5219-5227. Near linear scaling of the computational cost with the number of correlated electrons is achieved by applying domain and pair approximations. The PNOs are spin-independent and obtained using a semicanonical spin-restricted MP2 approximation with large domains of projected atomic orbitals (PAOs). The pair approximations of our previously described closed-shell PNO-LCCSD method are carefully revised so that they are compatible to the UCCSD theory, and PNO-UCCSD or PNO-RCCSD calculations for closed-shell molecules yield exactly the same results as corresponding spin-free closed-shell PNO-LCCSD calculations. The convergence of the results with respect to the thresholds and options that control the domain and pair approximations is demonstrated. It is found that large domains are required for the single excitations in open-shell calculations in order to obtain converged results. In general, the errors

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.

We have employed high-level coupled cluster methods including connected triple excitations to study the possibility of symmetry-breaking in the (2)B(2) ground state of the c-C(3)C(2)H radical. Specifically, we find that spin-restricted open-shell Hartree-Fock (ROHF) reference orbitals yield a C(2v) structure, whereas spin-unrestricted Hartree-Fock (UHF) and Brueckner orbitals lead to a symmetry-broken C(s) minimum-energy geometry. Equation-of-motion coupled cluster singles and doubles method for ionized states yields a C(s) structure with a double-zeta basis set, but not with a triple-zeta basis set. Through a detailed analysis of the orbital instability/near-instability behavior of each type of Hartree-Fock reference, we have determined that the UHF reference wave function is more reliable than the ROHF reference in this case and that the Born-Oppenheimer potential surface for c-C(3)C(2)H exhibits a symmetry broken C(s) global minimum. This result is supported by excited-state computations, which indicate that a second-order (pseudo) Jahn-Teller interaction is responsible for the symmetry-breaking.

The Lagrange-based Grassmann interpolation (G-Int) method has been extended for open-shell systems using restricted open-shell methods in building spin contamination-free potential energy surfaces.