# Basis Set Superposition Error (BSSE)¶

The Ghost Atom feature enables the calculation of Basis Set Superposition Errors (BSSE). Normally, if you want to know the bonding energy of system A with system B you calculate three energies

1. $$E(A+B)$$
2. $$E(A)$$
3. $$E(B)$$

The bond energy is then $$E(A+B) - E(A) - E(B)$$

The BSSE correction is about the idea that we can also calculate E(A) including basis functions from molecule B.

You can make a ghost atom by simply adding “Gh.” in front of the element name, for instance “Gh.H” for a ghost hydrogen , “Gh.C” for a ghost Carbon atom.

You will get a better bonding energy, closer to the basis set limit by calculating

$$E(A+B) - E(A/with B as ghost) - E(B/with A as ghost)$$

The BSSE correction is

$$E(A) - E(A/with B as ghost) + E(B) - E(B/with A as ghost)$$