Dynamically correlated reduced density matrices for density matrix downfolding

Develop one- and two-particle reduced density matrices that incorporate dynamic electron correlation for use in Density Matrix Downfolding, thereby improving the electronic-structure representation and the resulting effective Hamiltonians beyond the static correlation captured by complete active space configuration interaction.

Background

The paper obtains the reduced density matrices used in Density Matrix Downfolding from complete active space configuration interaction calculations, so the resulting quantities describe only static correlation within the active space. The authors state that an ideal fitting procedure would use reduced density matrices that also include dynamic correlation. Although methods such as NEVPT2 can recover dynamic correlation accurately at the energy level, the paper reports that implementations capable of producing the corresponding one- and two-particle reduced density matrices are not currently available. Developing such dynamically correlated reduced density matrices is therefore identified as an unresolved technical problem and a direction for future work in effective-Hamiltonian construction.