Supermolecular interaction energies

S.M. Cybulski, G. Chalasinski, R. Moszynski, On decomposition of second-order Mpller-Plesset supermolecular interaction energy and basis set effects. J. Chem. Phys. 92,4357 1363... 

To exploit this ability of the perturbative approach, some ab initio potential have been constructed by a separate fit of the various terms, as in the NEMO [75,107,108] potentials for water, partly based on Morokuma [25] decomposition of the supermolecular interaction energy, or the ASP-Wn potentials [45,109,110]. 

The severity of this can be diminished tty analyzing the supermolecular interaction energy expression (using molecular oibitals of A and B) and identifying the physically distinguishable terms by the kind of... 

The first term in the decomposition of the supermolecular interaction energy E s has the same formal expression as the corresponding term in the perturbation theory (Eq. 5). The basic expression for the use of the electrostatic potential as an index of chemical reactivity does not depend upon the theory adopted in describing molecular interactions. 

The correction contributes to the supermolecular interaction energy at the Hartree-Fock level. It is given by the following imple formula... 

An important drawback of the supermolecular approach is the low interpretative value of the results. This problem is also relevant for fitting model potentials. Several partition schemes have been proposed to enable physical interpretation of supermolecular interaction energies however. 

As is apparent from the above definitions, each of these effective matrices depend on basis sets and molecular orbitals of both fragments. It is also important to observe that these matrices possess a correct asymptotic behavior as at large interfragment distances they become the usual overlap and Fock matrices of the separate fragments, while the paired secular systems uncouple and converge to the separate Roothaan equations for the single monomers. Finally, as it is usual in a supermolecular approach, the interaction energy is expressed as... 

The potential energy surface for HeBr2 complex is examined using the supermolecular approach. In a supermolecular calculation, the interaction energy between a pair of atoms or molecules, is given by... 

Within the supermolecular approach, the total intermolecular interaction energy (AE) of a complex A---B is evaluated as the difference between the energy of the complex (EA" B) and the sum of the energies of its subsystems (EA, EB) ... 

Contrary to the previously described supermolecular approach, perturbation theory treatment allows for the partition of the interaction energy into physically interpretable components. The most frequently used method for this purpose is symmetry-adapted perturbation theory (SAPT) [13]. More recently, great effort has also been invested in the development of DFT-SAPT [14-16], In the present contribution, we use the variational-perturbational scheme [17-20], In this approach, the intermolecular interaction energy components are determined based on the wave functions of the subsystems evaluated in the dimer-centered basis set. Thus, both interaction energy and its components are BSSE-free. More details about this scheme can be found elsewhere [21-23]. The total intermolecular interaction energy at the MP2 level of theory can be expressed as follows ... 

G. Chalasinski, M. Szczesniak. On the connection between the supermolecular M0ller-Plesset treatment of the interaction energy and the perturbation theory of intermolecular forces. Mol. Phys. 63, 205-224 (1988)... 

In the variational method (also known as supermolecular method), the interaction energy A B is calculated from... 

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