AbstractThe character and the magnitude of magnetic interaction in 1‐D stacks of mixed molecular radical crystals (MMRCs) and mixed molecular ion radical crystals (MMIRCs) of alternant aromatic hydrocarbons have already been studied theoretically. In this work nonalternant 1‐D stacks built of π‐electronic subunits are considered. Some of the elementary units are singly or doubly charged (MMIRCs), others are neutral (MMRCs); alternant and nonalternant subunits are used as building blocks. Qualitative investigations based on the band theory of exchange effects in solid organic open‐shell systems are carried out. The contributions to the effective exchange integral Jeff: direct (Hund's), kinetic and indirect, are calculated. Structures of 1:1 MMRCs and MMIRCs with positive value of the integral Jeff, accounting for the spin exchange of the electrons in the half‐filled band, i.e., with ferromagnetic ground state, are suggested. The influence of different parameters (molecular size, presence of heteroatoms, localization versus delocalization, alternant versus nonalternant, charge, etc.) on the magnitude of the net exchange is discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004