Isolated Supermolecules

The first result to comment about is the evidently different description obtained for the isolated and the solvated supermolecules. In absorption, the result obtained in the isolated supermolecule is even blue-shifted with respect to cyclohexane, while in emission, the red-shift is recovered, but is very small. Only by adding the effect of the bulk with the PCM, the correct behavior is obtained with a net red-shift both in absorption and emission. On the emission, however, the correction introduced with the two explicit water molecules is not sufficient to get a real quantitative agreement with the experiments. Once again, a possible source of inaccuracy is the TDB3LYP description of the excited state geometry, which is here complicated by the presence of the H-bonded water molecules. 

Bixon, M., Jortner, J., Charge Separation and Recombination in Isolated Supermolecules, J. Phys. Chem. 1993, 97, 13061 13066. 

At infinite separation, one arrives at two boron atoms each having a donut-like cylindrical density as indicated in Figure 5-3. However, such a density cannot be obtained from real atomic p-orbitals. In other words, the density that results from the supermolecule is simply inaccessible from calculations on the isolated atoms. Whatever we do, we will never generate the correct charge density (and therefore energy) of the dissociated B2 molecule by calculations of the isolated boron atoms and the requirement of size-consistency is violated. Only if one switches to complex orbitals such as lpx rpyl, are cylindrical atomic densities possible. But even then, we are still in trouble and face a different problem. Just as... 

We have recently developed a new symmetry correlation scheme that connects symmetry-distinct rovibronic states of the isolated reactants with distinct electronic symmetries of the system in the interaction region independent of the detailed geometry of the collision complex. Briefly, the approach can be logically divided into four steps. First, the appropriate PI group for describing the isolated reactants is chosen. Second, P rvet reactant supermolecule system and the electronic... 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

With the success of these calculations for isolated molecules, we began a systematic series of supermolecule calculations. As discussed previously, these are ab initio molecular orbital calculations over a cluster of nuclear centers representing two or more molecules. Self-consistent field calculations include all the electrostatic, penetration, exchange, and induction portions of the intermolecular interaction energy, but do not treat the dispersion effects which can be treated by the post Hartree-Fock techniques for electron correlation [91]. The major problems of basis set superposition errors (BSSE) [82] are primarily associated with the calculation of the energy. 

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