The spin-polarized extended Brueckner orbitals.

摘要: Conventional natural and Brueckner orbitals (BOs) are rather frequently used for improving active orbital spaces in various configuration interaction (CI) approaches. However, the single-determinant models per se fail to give an adequate picture of highly correlated quasidegenerate states such as open-shell singlet dissociative states. We suggest use spin-polarized extended BOs formally defining them same manner Lowdin's spin-extended Hartree-Fock method. Such BO turn out be quite flexible particularly useful analyzing electronic It is shown that always exist, unlike usual unrestricted BOs. discuss difficulties related violation size-consistency spin projected determinant models. The working algorithm proposed computing within full CI complete space methodology. analyzed terms special density-like matrices associated with spin-up spin-down orbitals. From these density matrices, corresponding spin-polarization diagrams produced effectively unpaired (essentially correlated) electrons. illustrate approach by calculations on cyclic hydrogen clusters (H4, H6, H8), certain carbene diradicals monoradicals, low-lying excited computations show spin-projected provides a strong overlap multi-configurational state even bond breaking processes.