As first shown by Kollmar and Staemmler, and recently re-emphasized by Pernal and co-workers, the Δ𝐸 ST can be approximated by a combination of two terms, the first one being the exchange interaction 2𝐾 (given by Equation ) and a correction term Δ𝐸 DSP due to the DSP, which can be approximated with perturbation theory: 3