Which Ab Initio Wave Function Methods Are Adequate for Quantitative Calculations of the Energies of Biradicals? The Performance of Coupled-Cluster and Multi-Reference Methods Along a Single-Bond Dissociation Coordinate.

摘要: We examine the accuracy of single-reference and multireference correlated wave function methods for predicting accurate energies potential energy curves biradicals. The biradicals considered are intermediate species along bond dissociation coordinates breaking F-F in F2, O-O H2O2, C-C CH3CH3. apply a host approximations consistent way to same cases provide better assessment their relative accuracies than was previously possible. most method studied is coupled cluster theory with all connected excitations through quadruples, CCSDTQ. Without explicit quadruple excitations, obtained by RCCSDt method, followed, order decreasing accuracy, UCCSDT, RCCSDT, UCCSDt, seven methods, including perturbation theory, configuration interaction, coupled-cluster (with MRCI+Q being best Mk-MR-CCSD least accurate), four CCSD(T) then CCSD.