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Symmetry-adapted perturbation theory or SAPT is a methodology in electronic structure theory developed to describe non-covalent interactions between atoms and/or molecules. SAPT is a member of the family of methods known as energy decomposition analysis. Most EDA methods decompose a total interaction energy that is computed via a supermolecular approach, such that:
where Δ E i n t {\textstyle \Delta E_{\rm {int}}} is the total interaction energy obtained via subtracting isolated monomer energies E A {\textstyle E_{\rm {A}}} and E B {\displaystyle E_{\rm {B}}} from the dimer energy E A B {\textstyle E_{\rm {AB}}}. A key deficiency of the supermolecular interaction energy is that it is susceptible to basis set superposition error.
The major difference between SAPT and supermolecular EDA methods is that, as the name suggests, SAPT computes the interaction energy directly via a perturbative approach. One consequence of capturing the total interaction energy as a perturbation to the total system energy rather than using the subtractive supermolecular method outlined above, is that the interaction energy is made free of BSSE in a natural way.