A multicentered integrated quantum mechanical (MC QM/QM) scheme that can accurately treat extended it systems is presented. The trimers of cyanogen (N&3bond; C-C&3bond; N) and diacetylene (H-C&3bond; C-C&3bond; C-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 71 systems and other weakly bound clusters. (c) 2005 Elsevier B.V. All rights reserved.