Simple rules for predicting nonbonded interactions have been proposed. It was found that an end-to-end nonbonded interaction is either attractive or repulsive depending on the sign of the product of AO coefficients of two end atoms in the HOMO of a closed shell conjugated system with a crowded structure. The nonbonded attraction becomes the greatest in a 4N + 2 electron conjugated system, while it is repulsive in a 4N electron system. For 4N + 1 and 4N - 1 electron systems, it is attractive but the effect is less than that in 4N + 2 system. As a result of the attractive interaction, the overlap population of an atom pair increases (decreases) if the HOMO is antibonding (bonding) for the atom pair. The rules were illustrated with some examples.