Unrestricted coupled-cluster singles-doubles

In this research, the coupled-cluster single and double substitutions method with a perturbative treatment of triple excitations CCSD(T) [16-18] was adopted based on unrestricted Hartree-Fock reference wave functions. For the CCSD(T) computations, the cc-pVDZ, cc-pVTZ, and cc-pVQZ quality basis sets were used, where cc-pVnZ is an abbreviation for the correlation-consistent polarized valence basis sets of Dunning et al. [19-22]. With the coupled-cluster methods used in this research, the core orbitals are frozen. That is, the Is-like molecular orbital is frozen for O, while the Is2s2p3s3p3d-like molecular orbitals are frozen for Br. 

A time-dependent coupled cluster theory with unrestricted electron spins and full treatment of orbital rotation has been implemented to calculate the polarizabilities and dispersion coefficients. Illustration calculations on Li, Ar, HCl, CO, N2, O2, and H2O at the coupled cluster singles and doubles level have demonstrated the reliabihty of the method. Comparisons with HF and MP2 results have further shown the importance of high-order electron correlation effects whereas basis sets of the aug-cc-pVXZ family have been compared. 

QCISD(T) = quadratic Cl including single, double, and triple excitations, UCCD(ST) = coupled-cluster doubles method based on the unrestricted Hartree-Fock method and corrected for single and triple replacements, MC SCF = multiconfiguration SCF, MRD Cl = multireference singles and doubles Cl, MBPT= many-body perturbation theory, SD Cl = singles and doubles Cl. 

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