Nonbonding Molecular Orbital Method and Mathematical Proof for Disjoint/Non-disjoint Molecules

In this chapter, we introduce our own technique, which was developed to design non-disjoint systems with high-spin states more stable than those of disjoint systems, as mentioned in Chap. 1. Pure hydrocarbon (HC) systems are addressed first, because high-spin organic molecules are accessible when non-bonding molecular orbitals (NBMOs) are present as a result of the symmetry of the alternant HC skeleton. This description is then followed by a discussion of heteroatom-included HC (HHC) systems. We explain the basic idea of making non-disjoint combinations between molecules feasible when designing large organic non-disjoint systems with high-spin ground states.