The linear response function as a descriptor of non-covalent interactions: hydrogen and halogen bonds

An extension of the use of the linear response function to interpret non-covalent interactions is put forward. Due to its computational intricacies, most applications until now have been done on isolated atoms or molecules using coupled perturbed Hartree-Fock or Kohn-Sham theory, thereby adopting the simplest level for the LRF evaluation, the independent particle approximation. The previously presented possibilities for extension (the random phase and the "full" expression) are scrutinised, thereby highlighting the intricacies in the evaluation of the exchange-correlation term in the case of meta-GGAs and hybrid functionals, and implemented. A set of 25 hydrogen bonded and 11 halogen bonded systems, selected from Hobza S66 and X 40 compilations, were used to investigate the correlation between the stabilisation energy due to these non-covalent interactions and the relevant atom-atom-condensed LRF matrix element. The lack of a relevant correlation in the case of hydrogen bonding is contrasted with the excellent result for the halogen bonds. The correlation between the full option and the IPA is high providing support for our previous work using the IPA as is also the case for the previously used iterative Hirshfeld condensation and the more advanced FOHI method making use of fractional occupation numbers. The fundamental difference between hydrogen and halogen bond behaviour and the retrieval of the stability sequence within the halogen bonds series are traced back to the nature of the LRF as a response function for perturbations in the external potential putting polarisation effects and the polarisability of the atoms of the donor-acceptor couple at the forefront. The extension to the use of the softness kernel is advocated and already invoked to rectify the behaviour of two deviating complexes involving S as second row halogen bond acceptor atom.