In this study we examine the electronic and molecular structures of the [5(1) knot center dot center dot center dot(PF6)(10)](+) pentafoil knot system and report calculated interaction energies that result from halides (X = F, Cl, Br, and I) localized at the center of the [5(1) knot center dot center dot center dot(PF6)(9)]X molecular structure. The equilibrium geometries were fully optimized at the ONIOM(M06/6-31G(2d,p):PM6) level of theory, starting from an initial geometry for the pentafoil knot obtained from experimental X-ray data. The molecular systems were divided into two layers, for which the M06/6-31G(2d, p) level of theory was used to describe the high layer ([C4H6](5)X-structure) and the PM6 semiempirical method was employed for the low layer. The calculated electronic energies show that the interaction between the fluorine anion and the pentafoil knot produces the most stable structure, whereas an unfavorable interaction is observed for iodide due to the diffuse character of its electronic cloud. Using basis set superposition error (BSSE) correction techniques, the observed values of the interaction are -0.201 hartrees for the fluoride ion and -0.100 hartrees for iodide.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Univ Estadual Paulista, Fac Ciencias Bauru, Dept Quim, BR-17030360 Bauru, SP, BrazilUniv Estadual Campinas, Inst Quim, BR-13083970 Campinas, SP, BrazilUniv Estadual Paulista, Fac Ciencias Bauru, Dept Quim, BR-17030360 Bauru, SP, Brazil