Supermolecule method, interaction energy

We comment now on the most widely used method of calculation of intermolecular interaction energies which is based on the supermolecule... 

The basic strategy of the method was proposed by Wormer and van der Avoird who first applied it to the case of the He-He interactionThey showed that very simple VB wavefunctions provided the self-same interaction energies as fairly sophisticated MO-CI (supermolecule) calculations. Of... 

In the supermolecular approach the interaction energy AE of a complex is obtained by evaluating the total energy E b of the AB supermolecule using method X (X = SCF, Cl, CEPA, MPn, CPF,...) and subtracting the energy of the monomers ... 

In order to evaluate the accuracy of the tmncated 2 - - 3-body approach employed in this study, supermolecule LMP2 calculations with the mixed cc-pVDZ/aug-cc-pVDZ basis set were carried out on 200 structures selected from the apVDZ simulation trajectory. A comparison of the energies from the two methods is presented in Fig. 35.2, from which it is seen that the errors in the total energies due to the use of the 2 - - 3-body approach are relatively small. The average absolute error is calculated to be 0.22 kcal/mol, which is less than 0.7% of the total interaction energy. The largest error in the 2 - - 3-body energy... 

As a general remark, in the calculations of the intermolecular interactions using the supermolecule approach, the size-extensivity of the methods applied is of crucial importance. Furthermore, the interaction energies calculated in the supermolecule approach usually suffer from what is called the basis set superposition error (BSSE), a spurious energy improvement resulting from the use of truncated basis sets. This error seems to be unavoidable in most practical calculations except for very small systems. ... 

Most popular in the ab initio calculation of intermolecular potentials is the so-called supermolecule method, because it allows the use of standard computer programs for electronic structure calculations. This method automatically includes all the electrostatic, penetration and exchange effects. If the calculations are performed at the SCF (self-consistent field) level the induction effects are included, too, but the dispersion energy is not. The latter, which is an intermolecular electron correlation effect, can be obtained by configuration interaction (Cl), coupled cluster (CC) calculations or many-body perturbation theory (MBPT). These calculations are all plagued... 

As noted when introducing the Boys-Bernardi CP correction above, BSSE is a concern whenever the supermolecule method is used to compare the energies of fragments to the energy of the entire cluster [i.e., when computing the dissociation (Dg) or interaction (Tint) energy]. 

The second way to obtain the interaction energy is the more traditional method that is sometimes called the supermolecular approach. In this method, the interaction energy of, say, a dimer AB (the supermolecule) is obtained by directly subtracting the energy of the isolated monomers, A and B, from the total energy of the dimer, as follows ... 

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