  The method of increments (MoI) allows one to successfully calculate cohesive
energies of bulk materials with high accuracy, but it encounters difficulties
when calculating whole dissociation curves. The reason is that its standard
formalism is based on a single Hartree-Fock (HF) configuration whose orbitals
are localized and used for the many-body expansion. Therefore, in those
situations where HF does not allow a size-consistent description of the
dissociation, the MoI cannot yield proper results either. Herein we address the
problem by employing a size-consistent multiconfigurational reference for the
MoI formalism. This leads to a matrix equation where a coupling derived by the
reference itself is employed. In principle, such approach allows one to
evaluate approximate values for the ground as well as excited states energies.
While the latter are accurate close to the avoided crossing only, the ground
state results are very promising for the whole dissociation curve, as shown by
the comparison with density matrix renormalization group (DMRG) benchmarks. We
tested this two-state constant-coupling (TSCC)-MoI on beryllium rings of
different sizes and studied the error introduced by the constant coupling.
