# Exchange Energy Term in SAPT calculation

Is the Exchange Energy term in SAPT calculations the combination of both repulsive and attractive energies ? or just the exchange repulsion energy ?

You’ll be able to tell through examination of the “SAPT Results” table at the end of the job. http://psicode.org/psi4manual/master/sapt.html#interpreting-sapt-results

I understand that if my net Exchange energy value is positive it means repulsive and if its negative then its attractive . The Exchange energy has two types (Repulsive and Attractive) . Is SAPT taking into consideration both the repulsive and attractive components or it’s just summing up the repulsive components?So is Exchange Energy = Attractive Exchange Energy + Repulsive Exchange Energy OR
Exchange Energy = Repulsive Exchange Energy only? Hope my question is clear now .

Thank you

There are many term groupings and summary phases present in SAPT literature. By printing individual terms in that table (of which you’ll notice all the exchange are positive), you can map the Psi4 results onto the scheme you’re aiming for. To clarify expectations, you might take a system that has “Attractive Exchange Energy” in your scheme, run it through Psi and examine the table.

What do you mean by this " you can map the psi4 results onto the scheme you are aiming for" ?
Can you explain in detail please ?

Thank you

What I mean is that the table has the raw data and the docs (or lit citations) contain mathematical forms for those terms. I don’t understand your term “Attractive Exchange Energy”, but that’s fine because you do. With the definitions of the table’s terms and your mathematical understanding of what terms you seek, you can determine the correspondence.

In SAPT exchange is always positive as we are looking at Pauli exchange not Slater as found in various DLPNO decompositions.

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