PNNL

Abstract

Projection-based embedding theory (PBET) is used to calculate and assess the challenging spin-crossover energies for a selection of small Fe-containing systems by embedding the metal center into the frozen potential of the ligands. MP2, CCSD, and CCSD(T) are embedded in potentials from the SCAN and r2SCAN functionals and compared with the canonical values for the constituent methods and previously reported reference values. In comparison to reference values back-corrected from experimental results, PBET is shown to consistently improve the accuracy of the spincrossover energy as compared to the corresponding wave function method for the whole system, and improves over the DFT methods for some of the systems. For the systems with spin-crossover values near zero, the wave function methods perform quite poorly and the PBET serves to improve the results to some extent, but at the cost of the accuracy already seen in the DFT results. Compared to higher accuracy wave function theory simulation of several complexes, CCSD(T)-in-DFT is able to deliver equal or improved results compared to both the isolated CCSD(T) and DFT quantities for the same complexes. The isolated recalculation of the electronic structure around the metal center when embedded into a DFT treatment of the ligand field shows promise as a pragmatic and lower cost treatment compared to the canonical treatment of the whole system of the difficult class of spin-crossover complexes.