State-Averaged Density Matrix Embedding Theory for Local Excitations

Density matrix embedding theory (DMET) provides an elegant framework in quantum chemistry to describe local properties of chemical systems, which allows a high-level method to solve an embedded subsystem constructed based on a low-level treatment of the whole system and therefore achieves a balance between efficiency and accuracy. However, because the embedded subspace in DMET is typically constructed from the mean-field ground-state Slater determinant, the resulting bath orbitals inherently favor the ground state, leading to unbalanced descriptions of ground and excited states for local excitations. In this work, we first demonstrate the starting point dependence of DMET in excitation energy calculations and then generalize the original ground state-based DMET by extending the starting point from the single Slater determinant to the state-averaged (SA) complete active space self-consistent field (CASSCF), hence termed as SA-DMET. In calculations of magnetic anisotropy and excitation energies of transition metal and lanthanide complexes, SA-DMET shows significant improvement in accuracy compared to the single-state DMET. Configuration-averaged Hartree-Fock (CAHF), which is equivalent to SA-CASSCF when all states in the chosen active space are equally averaged, is found to give comparable accuracy as a DMET starting point, thus offering a more efficient choice for state-averaged embedding. Finally, the recently proposed nonorthogonal atomic orbital-based DMET (AO-DMET) is tested on various systems and gives very promising results in all cases. These results establish SA-DMET, especially in combination with AO-DMET and CAHF, as a robust and efficient embedding framework for local excited states in strongly correlated metal complexes.