UHF-CCSD

P. G. Szalay, J. Gauss, and J. F. Stanton, Theor. Chim. Acta, 100, 5 (1998). Analytic UHF-CCSD(T) Second Derivatives Implementation and Application to the Calculation of the Vibration-Rotation Interaction Constants of NCO and NCS. 

Note Geometry, basis set, and full Cl results taken 60m Bauschlicher and Taylor (35). QRHF-CC results for 1 At ionizations from Watts and Bartlett (26). 3Bi calculations used a UHF-CCSD reference function. 

Three different experimental electron affinity values have been reported for C4 (3.7, 3.88, and 3.79 eV). Exactly which measure of EA is being sampled is debatable, but it appears to be closest to AEA. The higher precision value of 3.88 eV would appear to offer the best comparison. Allowing for a correction of 0.15 eV from VEDE values as obtained in the PVTZ UHF-CCSD(T) results, the larger, more diffuse basis values cluster near 3.8 eV for this quantity. 

Likewise, when high spin contamination (see Spin Contamination) results in a high T in an UHF-CCSD(T) calculation, anharmonicities can be seriously in error. For example, for BC(X E ), UHF-CCSD(T)/cc-pVTZ produces cOgXe = 28.2 cm and = 0.02413 cm compared with CAS-ACPF/cc-pVTZ values of 10.2 and 0.01628 cm respectively. In such situations, ROHF-CCSD(T) may offer a solution at the ROHF-CCSD(T)/cc-pVTZ level, we find (OgXe = 10.2 cm and ag = 0.01672 cm , which suggests the problem was indeed solely related to spin contamination. Unusually poor agreement with experiment was likewise seen for UHF-CCSD(T) harmonic frequency calculations on different structures of BN2, where spin contamination is quite prominent. 

Table 5 Spin Contamination in UHF-CCSD and ROHF-CCSD Calculations Monitored by the Expectation Value for S. For Further Details, see Ref. 68...

Table 6 Number of Independent Amplitudes for the Closed-shell (RHF) and Open-shell (UHF) CCSD Approaches with n Denoting the Number of Occupied Spin Orbitals and N the Number of Virtual Spin Orbitals...

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