2020 Volume 14 Issue 1 Pages A0109-
In non-relativistic fermionic systems, spin is a vital property. Approximate ground state wave functions can often violate spin-symmetry by relaxing spin constraints to provide additional electron correlation effects essential for describing multi-determinant systems such as transition metal complexes. Symmetry breaking, however, comes at the price of losing good quantum numbers and deteriorating molecular properties. We describe the projection technique to restore the lost symmetry, which can be combined with traditional electronic structure theories, delivering higher accuracy in the treatment of dynamical and static electron correlations.
