## Tuning of non-covalent interactions involving a halogen atom that plays the role of Lewis acid and base simultaneously

In this work, a series of model complexes (MH_{3}X)(HNC)(N'C’H’), where M = C–Pb and X = Cl–At, is studied using a first-principles computational approach. Each of these complexes possesses a halogen atom (X) simultaneously acting as the Lewis base for X···H hydrogen bonding (HB) with HNC and as the Lewis acid for X···N’ halogen bonding (XB) with N'C’H’. The strengths of these non-covalent interactions are tuned by sharing the same halogen centre and by substituting M and X with consecutive elements from groups 14 and 17. Variations in the strengths are estimated mostly by means of various energetic quantities, such as the total interaction energy (*E*_{int}), two- and three-body contributions to *E*_{int}, their fundamental physical components and donor–acceptor orbital interaction energies. The coexistence of HB and XB involving the same X-centre weakens these interactions, but the magnitude of the total interaction in the complexes increases due to the three-body interaction and the appearance of additional lateral non-covalent interaction between HNC and N'C’H’. Substituting M and X with consecutive elements from groups 14 and 17 leads to several regularities in the changes of *E*_{int}. A dependence of three-body interaction on the kinds of M and X is also detected.