Electrostatic exchange-overlap repulsion

Explicit calculations on the dimer are only required for the exchange-repulsion term, which is obtained as the difference between the SCF energy of the supermolecule and electrostatic and induction terms. To fit the exchange-repulsion term, on the other hand, functions only dependent on overlap integrals of unperturbed SCF wavefunctions of the monomers are used. 

In fig. 5 we see that, indeed, the T-shaped and the staggered parallel structure have maximum electrostatic attraction. The dispersion energy is most favourable, of course, for the linear structure. For distances in the neighbourhood of the (isotropic) Van der Waals minimum the (short range) exchange repulsion is the dominant anisotropic term, however. Since it increases very steeply when the molecular charge clouds start to overlap (especially in the linear structure 0 = 0g = 4>a — 1... 

Atoms with closed subshells have no multipole moments and their electrostatic and induction interactions have a pure overlap origin from which follows their short-range character. The main contribution to Ef F gives the exchange interaction ch- Between atoms with closed subshells, it is repulsive (as in the noble-gas atom systems). This determines the unstability of the alkaline earth dimers at the SCF approximation. They are stabilized by the attractive electron correlation forces. 

Like their related methods such as the Intermediate Neglect of Differential Overlap (INDO) [2] and Modified Intermediate Neglect of Differential Overlap (MINDO) [6] the methods already mentioned have a number of features in common. They are all self-consistent field (SCF) methods that take into account electrostatic repulsion and exchange stabilization and where all integrals are evaluated by approximate means. They are further characterized by the fact that they all use a restricted basis set, that is, one s oibital and three p orbitals per atom, except for the hydrogen atom, which is represented by an s orbital. 

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