Molecular orbital energetic ordering

Thus, both symmetry and energy requirements must be fulfilled in order to form molecular orbitals. Energetically, the 2s and 2p atomic orbitals are sufficiently similar to form molecular orbitals with each other. For symmetry... 

This way, based on the symmetry groups theory and their representations, and in particular by the SALC exposed procedure, it can be formed a quantum picture of the molecular bonds, without approximations, but also without the possibility of energetic ordering of the obtained orbitals and without the capacity to assess the real order of combination of atomic orbitals in the molecular orbitals. Energetic and hybridization informations of... 

There is another physical phenomenon which appears at the correlated level which is completely absent in Hartree-Fock calculations. The transient fluctuations in electron density of one molecule which cause a momentary polarization of the other are typically referred to as London forces. Such forces can be associated with the excitation of one or more electrons in molecule A from occupied to vacant molecular orbitals (polarization of A), coupled with a like excitation of electrons in B within the B MOs. Such multiple excitations appear in correlated calculations their energetic consequence is typically labeled as dispersion energy. Dispersion first appears in double excitations where one electron is excited within A and one within B, but higher order excitations are also possible. As a result, all the dispersion is not encompassed by correlated calculations which terminate with double excitations, but there are higher-order pieces of dispersion present at all levels of excitation. Although dispersion is not necessarily a dominating contributor to H-bonds, this force must be considered to achieve quantitative accuracy. Moreover, dispersion can be particularly important to geometries that are of competitive stability to H-bonds, for example in the case of stacked versus H-bonded DNA base pairs. ... 

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