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Summary of the Computational Tests

Some BOVB-calculated bonding energies in double-zeta polarized basis sets [Pg.215]

Finally, a few remarks are in order concerning the non dynamic correlation of the inactive electrons. Normally, these electrons are left uncorrelated (except in the extended SD-BOVB calculation for H-F, above) in the molecule as well as in the dissociated fragments or in any conformation of a molecular system throughout a potential surface. However, since the inactive orbitals are somewhat different in the HL and ionic VB structures, it is impossible to avoid [Pg.216]

The energy collapse due to spurious correlation of inactive orbitals may be exceptionally encountered, even if the active orbitals are not delocalized, as has been observed for ZnH+ above [36]. Such an artefact is however easy to detect, based on the fact that an inactive pair in an ionic structure occupies an orbital that is mostly virtual in the HL structure, e.g. an orbital displaying a node. The remedy consists of effectively giving the inactive electrons the level of correlation that they try to achieve. This can be done by going to the extended SD level as in FH above, however this rigorous solution makes the calculation rather cumbersome. A much easier corrective procedure is to double the major VB structure at any point of a potential surface all the way to the dissociated products, if any. In this way, the excess stabilization of the inactive orbitals carries over to the whole potential surface, which deletes any artefactual overbinding effect. This procedure has been used successfully in the ZnH+ case. [Pg.217]

One of the most valuable features of theoretical methods based on classical VB structures is their ability to calculate the energy of a diabatic state. For practical uses, some diabatic bond energy curves of chemical interest can be, for example, the separate dissociation energy curves of the ionic and covalent components of a bond, or the energy curves of the effective VB structures of a [Pg.217]

While the definition of an adiabatic state is straightforward, as an eigenfunction of the Hamiltonian within the complete set of VB structures, the concept of diabatic state is less clear-cut and accepts different definitions. Strictly speaking, a basis of diabatic states (fi, ...) should be such that eq 21 is satisfied for any variation 5Q of the geometrical coordinates. [Pg.218]


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