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We employ natural bond orbital and natural resonance theory tools to analyze the enigmatic properties of the C_2v-symmetric isomer of chlorine dioxide radical (ClO₂), whose many challenges to Pauling-type localized bonding concepts were recognized by Linus Pauling himself. Although spin-contamination is minimal in this species, ClO₂exhibits an unusually strong form of “different Lewis structures for different spins” bonding pattern, intrinsically outside the framework of “maximal pairing” concepts. We show how the novel spin-unpaired donor–acceptor interactions lead to weakened bonding in the supramolecular domain of polyradical (ClO₂)_nhomoclusters and aqueous ClO₂(H₂O)_nheteroclusters. Despite feeble binding energies and large inter-radical separations, the polyradical clusters are found to maintain coherent spin patterns in each cluster component, attesting to the quantal donor–acceptor nature of their interactions and the cooperative and anticooperative couplings that govern intra- and intermolecular spin distributions in such spin-clusters.

We describe the formal algorithm and numerical applications of a novel convex quadratic programming (QP) strategy for performing the variational minimization that underlies natural resonance theory (NRT). The QP algorithm vastly improves the numerical efficiency, thoroughness, and accuracy of variational NRT description, which now allows uniform treatment ofallreference structures at the high level of detail previously reserved only for leading “reference” structures, with little or no user guidance. We illustrate overall QPNRT search strategy, program I/O, and numerical results for a specific application to adenine, and we summarize more extended results for a data set of 338 species from throughout the organic, bioorganic, and inorganic domain. The improved QP‐based implementation of NRT is a principal feature of the newly releasedNBO 7.0program version. © 2019 Wiley Periodicals, Inc.

We provide a didactic introduction to 2nd‐quantized representation of complex electron–hole (e/h) excitation patterns in general configuration interaction wave functions built from orthonormal local orbitals of natural atomic orbital or natural bond orbital (NBO) type. Such local excitation patterns of chemically oriented basis functions can be related to the resonance concepts of valence bond theory, and quantitative evaluation of the associated excitation probabilities then provides an alternative assessment of resonance “weighting” that may be compared with those of NBO‐based natural resonance theory. We illustrate the usefulness of anticommutation relations in deriving Pauli‐compliant expressions for allowed excitation patterns, showing how the exciton‐like promotions φ_λ → φ_ν(creating ane/hexcitation withhin φ_λandein φ_ν) impose strict constraints on associatede/h‐probabilities (requiring, e.g., that thee‐probability for an electron “to be” or “not to be” in φ_νmust be rigorously linked to the complementaryh‐probabilities in φ_λ). Specific examples are presented of the quantum Boolean logic for four or six local spin‐orbitals, with emphasis on Natural Poly‐Electron Population Analysis (NPEPA) evaluation of VB‐type covalent and ionic contributions in conventional 2‐center bonding, resonance weightings in 3‐center hydrogen bonding, and general characteristics of higher‐orderm‐center bonding motifs form > 3. Numerical results are presented for methylamine, acrolein, and water dimer to illustrate current NPEPA implementation in the NBO program. © 2019 Wiley Periodicals, Inc.