SCS-MP2 revisited: optimised spin-component scaling boosts accuracy for Halogen van der Waals complexes

Halogens are versatile in the bond formation due to their electron configuration and ability to participate in various types of interactions. Accurate description of halogen bonding often requires high-level quantum mechanical methods like CCSD(T), computationally prohibitive. This work reconsiders a well-established but underutilised framework: second-order Møller–Plesset perturbation theory (MP2). While standard MP2 provides a qualitatively correct description of halogenated systems, spin component scaling (SCS) of the correlation energy is known to significantly improve its quantitative accuracy. However, there is no MP2-based SCS methodology developed for systems with halogen atoms. Therefore, we propose an SCS-based approach to accurately describe interaction energies in halogenated systems, contained in the X40 and Bauzá databases (F, Cl, Br, and I). Notably, our findings reveal that, with properly optimised SCS-MP2 coefficients, the method proposed here is capable of predicting with a high degree of accuracy systems of the type: hydrogen bonds; dispersion-dominated complexes covering π−π, aliphatic-aliphatic and <i>π</i>-aliphatic interactions; complexes with mixed electrostatic/dispersion interaction. This type of interactions includes atoms of O, N, S y P Cl and F. The performance of our approach outperforms that obtained from methodologies developed to study weak interactions, such as the Symmetry-Adapted Perturbation Theory for DFT (SAPT(DFT)) and some density functional approximations. The SCS-MP2 method, introduced by Grimme over two decades ago, is now barely utilised for modelling weak interactions. However, the results of this study demonstrate that this approach deserves further exploration and development.