The quantum-mechanical entanglement of molecular fragments is approached within the wavefunction (WFT) and density-functional (DFT) theories. The natural (intra-fragment decoupled) representation of the effective density operators of the complementary molecular subsystems in the given molecular state is examined in some detail and holistic phenomena in such entangled quantum states are explored by comparing the effective density operators of the embedded subsystems and the statistical operator of the molecule as a whole. The “external” (inter-fragment) correlation energy is expressed in terms of the average correlation holes of DFT resulting from the coupling-constant scaling of the electron repulsion between molecular subsystems. The equilibrium analogs of the entangled states of subsystems are explored. They extremize the state resultant entropy combining the classical (probability) and nonclassical (phase/current) contributions. In the Harriman-Zumbach-Maschke construction of the subsystem equilibrium wavefunctions, the electron densities of molecular fragments, pieces of the molecular distribution, determine the state moduli while “thermodynamic” phases describing the mutually-closed (nonbonded, disentangled) or mutually-open (bonded, entangled) subsystems are determined by the fragment and molecular probability distributions, respectively. As an
illustration the hypothetical stages of a bimolecular reaction are examined and the current promotion of reactants in their equilibrium states is explored.
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