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Intermolecular perturbation first-order energy

The first-order energy vanishes in the present case since the geometry of the fragment A, and that of the fragment B as well, is assumed to be unaffected by intermolecular perturbation. Note that the first-order orbital mixing between and (i.e., f(l ) leads to the second-order energy change The second-... [Pg.32]

Considering only the interaction between HOMO of R and LVMO of S, elementary perturbation theory shows that the result of the orbital interaction is a repulsion of the levels, the occupied level becomes more stable, the unoccupied level less stable. The simplest Huckel-type formulation of PMO theory gives equations for the intermolecular perturbation energy change A that are quite simple in form, Eqs. 3—6 18,20,22,26-28) Q is a first-order Coulombic energy that can be calculated in terms of... [Pg.146]

In intermolecular perturbation theory one of the major problems concerns electron exchange between molecules. In the method described here exchange is limited to single electrons. This simplification is definitely a good approximation at large intermolecular distances. The energy of interaction between the molecules, AE (R), is obtained as a sum of first order, second order, and higher order contributions ... [Pg.17]

Using Eqs. (6) and (10) the first-order contribution to intermolecular perturbation energy can be derived without further difficulty 73>74> ... [Pg.18]

Before we tarn to MO theory of molecular interactions a short discussion on the reliability of semiempirical calculations of the CNDO type by means of perturbation theory would be useful. For a better understanding of the possibilities and limitations of semiempirical MO approaches to intermolecular forces we calculated first-order perturbation energies for very simple complexes with and... [Pg.21]

The simplest implementation of integral equation theory for blends where the intermolecular potentials include an attractive component is via first order perturbation theory. In this case the free energy. A, is... [Pg.2127]

In this approach the properties (e.g. distribution function or free energy) of a system with a given intermolecular potential F, are related to those of a reference system with potential Fq, usually by an expansion in powers of the perturbation potential Fj = F - Fq. The perturbation terms, first-order, second-order, etc., involve both F and the distribution function of the reference system (see, among others, Gubbins et al, and, especially. Gray and Gubbins, 1984). [Pg.631]

Chalasinski and Szczesniak have provided a means of decomposing the correlation contribution to the interaction energy into four separate terms. Their philosophy takes the electron exchange operator as a second perturbation in the spirit of many-body perturbation theory, with molecular interaction as the first perturbation in their intermolecular Mpller-Plesset perturbation theory (IMPPT). At the level of second order of the correlation operator, they obtain a number of separate terms. The first is the dispersion energy, e... [Pg.38]

The first attempt to develop a statistical model of the cholesteric phase was by Goossens who extended the Maier-Saupe theory to take into account the chiral nature of the intermolecular coupling and showed that the second order perturbation energy due to the dipole-quadrupolar interaction must be included to explain the helicity. However, a diflUculty with this and some of the other models that have since been proposed is that in their present form they do not give a satisfactory explanation of the fact that in most cholesterics the pitch decreases with rise of temperature. [Pg.298]


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