Integrated quantum mechanical approaches for extended π systems: Multicentered QM/QM studies of the cyanogen and diacetylene trimers

A multicentered integrated quantum mechanical (MC QM/QM) scheme that can accurately treat extended π systems is presented. The trimers of cyanogen (NC–CN) and diacetylene (H–CC–CC–H) are studied in seven geometries. MC QM/QM calculations reproduce CCSD(T) binding energies within 0.04 kcal mol−1. In contrast, MP2 substantially overestimates these interaction energies. An MC QM/QM version of the substituent value test is introduced, and the error in MC QM/QM results is discussed in this context. The results for these systems suggest that MC QM/QM may be a powerful tool in the study of extended π systems and other weakly bound clusters.