Dispersive interactions, charge transfer systems

The SMO-LMBPT method conveniently uses the transferability of the intracorrelated (one-body) parts of the monomers. This holds, according to our previous results [3-10], at the second (MP2), third (MP3) and fourth (MP4) level of correlation, respectively. The two-body terms (both dispersion and charge-transfer components) have also been already discussed for several systems [3-5]. A transferable property of the two-body interaction energy is valid in the studied He- and Ne-clusters, too [6]. In this work we focus also on the three-body effects which can be calculated in a rather straightforward way using the SMO-LMBPT formalism. 

Fortunately, there are a number of theoretical, statistical and empirical reasons to believe that Lewis basicity (affinity) depends on a limited number of factors. From the quantum chemical point of view, the acid/base interaction energy can be partitioned into five terms (electrostatic, dispersion, polarization, charge transfer and exchange-repulsion). By a principal component analysis [184], 99% of the variance of an afflnity/basicity data matrix can be explained by three factors, the first two being by far the most important. A number of experimental affinity and basicity scales, and of spectroscopic scales of basicity, can be correlated by two parameters, using the EC or equations, or three quantum chemical descriptors of basicity [197]. However, these statistical and empirical approaches are limited to systems where steric effects and tt back-bonding are not important. 

Table 1. Value of the energetic components of the interaction energy for the indicated systems, computed using the IMPT method. Ea, E, Ep, ct, and (jjsp are, respectively, the electrostatic, exchange-repulsion, polarization, charge-transfer, and dispersion components. tot is the sum of these components and Euvi is the BSSE-corrected interaction energy, shown to serve as reference to calibrate the quality of the IMPT calculation. The MP2 intermolecular optimum distance is also shown (ropt). Distances are given in A and energies in kcal/mol...

In numerous systems, a fundamental role is played by the van der Waals forces . Conformations of single molecules and intermolecular aggregations are strongly affected by van der Waals interactions which may be evaluated by quantum chemical methods. True van der Waals interactions are those when the dominant attractive contribution is the dispersion energythe term van der Waals complex may be considered better than charge-transfer complex when no charge is transferred during the complex formation. 

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