Energy Decomposition Analysis Method Based on the Constrained Active Space Unrestricted Mean-Field Method

An energy decomposition analysis method based on the constrained unrestricted mean-field (CUHF) theory, called GKS-EDA(CU), is presented for intermolecular interactions involving radicals. This method is able to take into account the spin contamination in both open-shell singlet states and high-spin states. By using GKS-EDA(CU), the total interaction energy can be divided into the terms of electrostatic, exchange-repulsion, polarization, and correlation. Test examples illustrate that GKS-EDA(CU) not only overcomes the spin contamination in the analysis for radical-radical interactions but also provides insights into the variation of the magnetic coupling constant.