Abstract
In order to set the benchmark data for X-π halogen bonds and to validate the performance of different theoretical methods for halogen bond energy, four kinds of X-π halogen bond systems (hydrogen halide-benzene, methyl halide-benzene, ethynyl halide-benzene, and phenyl halide-benzene) were calculated by CCSD(T) and other methods. To simulate the most abundant X-π halogen bond systems in biomolecules, the phenyl halide-benzene complexes were used. Other three kinds of model systems provided varied types of X-π halogen bond systems to prevent calculation methods from being specific for phenyl halide-benzene system. The benchmark data were established with the CCSD(T)/CBS method, which is referred to as the gold-standard method. Moreover, because of the high computational costs of the CCSD(T) method, a calculation methods that can reproduce benchmark data must be found. Thus, all the model systems were calculated using the MP2, SCS-MP2, DFT, and MP2.X methods, and then the bond energy results were compared with benchmark data. All the complexes were optimized by MP2/Def2-TZVPD. The CCSD(T)/CBS energy of chlorobenzene-benzene, bromobenzene-benzene, and iodobenzene-benzene system is 1.61, 2.05, and 2.76 kcal/mol, respectively. The bond energies calculated by the MP2 method are larger than benchmark data, while the SCS-MP2 method and the DFT method strongly underestimate the bond energies. Three MP2.X methods perform well in halogen bond energy calculation: MP2.5(avdz-avtz-avdz), MP2.5 (avdz-avtz-cvtz), and MP2.4(cvdz-cvtz-cvdz). These three methods have error less than 2% for all systems. Considering the computational costs, the MP2.4(cvdz-cvtz-cvdz) method is suggested as the most satisfactory method for halogen bond energy calculation.