The ghost keyword introduces ghost atoms into the calculation.  

The keyword is supported by all of the methods in the QuantumChemistry package.

Ghost atoms are ethereal in that they only contribute basis functions without adding a nucleus or any additional electrons.

Basis Set Superposition Error (BSSE) is the error from an overestimation of "weaker" interactions such as the interactions between molecules due to the incompleteness of the basis set.  The individual energies of two or more chemical moieties are lowered upon approach due to a borrowing of other moieties' basis functions to offset the incompleteness of the basis set.  The lowering is inconsistent or artificial because it arises from basis-set incompleteness rather than the chemical interactions.

A common approach to correcting BSSE is the counterpoise correction.  The counterpoise correction estimates the basis-set incompleteness by performing a series of calculations with and without ghost atoms.  The calculations with the ghost atoms allow for an approximate separation of the energy contributions from basis-set overcompleteness and chemical interactions.

$\mathrm{with}\left(\mathrm{QuantumChemistry}\right)\:$

$\mathrm{data} ≔ \mathrm{DensityFunctional}\left(\left[\left[''Ne''\,0\,0\,0\right]\right]\, \mathrm{basis}\=''cc-pvdz''\,\mathrm{ghost}\=\left[\left[''Ne''\,0\,0\,1.0\right]\right]\right)\;$