Analysis Using Symmetry-Adapted Perturbation Theory

Finally, we should also briefly discuss the performance of semiempirical methods. These are methods that neglect some of the more expensive integrals in Hartree-Fock molecular orbital theory and replace others with empirical parameters. Because semiempirical methods are based on Hartree-Fock theory, and because Hartree-Fock theory does not capture dispersion effects, semiempirical methods are not suitable for computing dispersion-dominated noncovalent interactions. Semiempirical methods yield repulsive potentials for the sandwich benzene dimer, just as Hartree-Fock does. However, given that semiempirical methods already contain empirical parameters, there is no reason not to fix this deficiency by adding terms proportional to r, as is done in force-field methods and the empirical DFT-D methods. Such an approach has been tested for some base pairs and sulfur-7t model systems. 

ANALYSIS USING SYMMETRY-ADAPTED PERTURBATION THEORY 

So far in this review, we have focused on various approaches to computing the strength and geometries of noncovalent n interactions. However, not only can computational chemistry provide such information, but it can also provide a means of analyzing that information. In studies of complex phenom-data analysis can be the most important step if it yields a rational 

Symmetry-adapted perturbation theory is a well-motivated theoretical approach to compute the individual components of intermolecular interactions, namely, the electrostatic, induction, dispersion, and exchange-repulsion terms. The approach is a double-perturbation theory that uses a Hartree-Fock reference, with a Fock operator F written as the sum of Fock operators for the individual molecules. Both the intramolecular correlation potential W) and the intermolecular interactions (V) are treated as perturbations, so that the Hamiltonian is expressed as 

The superscripts in Eq. [19] refer to orders in the two perturbations, V and W, respectively. The terms denoted resp refer to the inclusion of higher order terms via the coupled-perturbed Hartree-Fock equations. The term is a cross term involving both exchange and induction. The 

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Analysis Using Symmetry-Adapted Perturbation Theory 25... 

ANALYSIS USING SYMMETRY-ADAPTED PERTURBATION THEORY... 

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