Molecular interactions Pauli repulsion

The comparative analysis of the real-space subsystems emerging in fermionic and bosonic systems is another novel aspect of the present study. This is an interesting area for future studies since it may reveal the local role of the Pauli Exclusion Principle in molecular systems. Pauli repulsions and associated steric interactions are usually invoked in both qualitative and quantitative analysis to rationalize conformational selections, tracing molecular stresses and instabilities. However, most of such analyzes are based on indirect methods and one may hope that a direct comparative QTAIM analysis on a fermionic system and associated bosonic counterpart may reveal a more detailed picmre of the role of the statistics on the local interactions in molecular systems. 

Since no physisorption well is present, the question has to be considered how the inclusion of a physisorption state would alter the trapping dynamics. Physisorption wells are created by a combination of the attractive van der Waals interaction with Pauli repulsion caused by the overlap of molecular and substrate wave functions. While the former effect is not reproduced by the DFT calculation, the repulsion due to wave function overlap is well described by present DFT functionals. Hence, the calculated PES would only become more attractive if van der Waals forces were correctly included. For a more quantitative description of the trapping process at kinetic energies below 0.05 eV certainly the physisorption channel has to be included. 

EX Exchange energy. Part of the interaction energy between static charge clouds of two subunits, resulting from Pauli exchange between them. Similar to steric repulsion for molecular interactions. 

From a simple frontier orbital point of view the primary features of H—H bond activation are (1) attractive metal cluster donor interactions between the high-lying unoccupied H2 a antibonding orbital and the highest occupied molecular orbital M (HOMO) of the cluster, (2) attractive metal acceptor interactions between the occupied Hj ff bonding orbital and the lowest unoccupied cluster orbital M(LUMO), and (3) Pauli repulsion between the filled Hj ff orbital and filled cluster valence orbitals. 

As an illustration of this, the difference between the computed bond energies of F2, which is -260 kJ/mol, and Ng, which is -1000 kJ/mol, is analyzed. In the F2 molecule both the bonding and antibonding repulsive interactions. The only pair of molecular orbitals where electrons exclusively occupy only the bonding orbital is the <75 orbital constructed from the 2p2 atomic orbitals. This results in an overall attractive contribution to the chemical bond. The attractive orbital overlap contribution which is equal to 2A(2p ) =-9.1 eV is counteracted by the Pauli-repulsive interactions due the [Pg.87]

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