Question

When is it necessary to consider size consistency?

Answer 1

I understand that if a given theory for the evaluation of the energy is size consistent, then the energy of the supersystem A+B, separated by a sufficiently large distance so there is essentially no shared electron density, is equal to the sum of the energy of A plus the energy of B taken by themselves.

Which is particularly important in energy calculations for bond dissociation reactions, however is the concept of size consistency important when calculating energy of reaction for isodesmic and insertion reactions

In theory, size consistency is always a problem for numerical integration with finite numbers of basis functions. The 'type' of reaction is less relevant than the extent of additional variational flexibility afforded by the superposition of the basis sets of each system

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