NATURAL RESONANCE THEORY - I - GENERAL FORMALISM

We present a new quantum-mechanical resonance theory based on the firs t-order reduced density matrix and its representation in terms of natu ral bond orbitals (NBOs). This ''natural'' resonance theory (NRT) depa rts in important respects from the classical Pauling-Wheland formulati on, yet it leads to quantitative resonance weights that are in qualita tive accord with conventional resonance theory and chemical intuition. The NRT variational functional leads to an optimal resonance-weighted approximation to the full density matrix, combining the ''single refe rence'' limit of weak delocalization (incorporating diagonal populatio n changes only) with the full ''multireference'' Limit of strong deloc alization (incorporating off-diagonal couplings between resonance stru ctures). The NRT variational functional yields an error measure that s erves as an intrinsic criterion of accuracy of the resonance-theoretic description. The NRT program structure, algorithms, and numerical cha racteristics are described in supplementary material, and detailed che mical applications are presented in two companion papers. (C) 1998 Joh n Wiley & Sons, Inc.