Unrestricted Coupled Cluster and Brueckner Doubles Variations of W1 Theory

Unrestricted coupled cluster spin contamination corrected [UCCSD(T)] and unrestricted Brueckner doubles [UBD(T)] variations of the Weizmann-1 theory (W1), denoted as W1U, W1Usc, and W1BD, respectively, are compared with the restricted open-shell W1 theory [W1(RO)]. The performances of the four W1 variants are assessed with 220 total atomization energies, electron affinities, ionization potentials, and proton affinities in the G2/97 test set, for consistency with the error analysis of the original W1 (RO) study. The root-mean-square deviations from the experiment of W1U (0.65 +/- 0.48 kcal/mol), W1Usc (0.57 +/- 0.48 kcal/mol), W1BD (0.62 +/- 0.48 kcal/mol), and W1(RO) (0.57 +/- 10.48 kcal/mol) show that the four methods are virtually indistinguishable. This error analysis excludes the singlet biradicals, C-2 and O-3, since single determinantal methods are not really adequate for these strongly multireference systems. The unrestricted W1 variants perform poorly for such highly spin-contaminated and multireference species (the largest deviation from experiment for W1Usc is -4.2 +/- 0.1 kcal/mol for the O-3 EA). W1(RO) performs much better than its unrestricted counterparts for these pathological cases (the deviation from experiment is reduced to -1.5 +/- 0.1 kcal/mol for the O-3 EA), though the errors are significantly larger than those for the overall test set. The examples Of C-2, O-3, and the F-2 potential energy curve indicate that an advantage to using W1BD is that the error in < S-e > correlates with the magnitude of the error in energy, whereas W1 (RO) loses accuracy without such a warning.