Basis Set Superposition Error (BSSE)
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The Ghost Atom feature enables the calculation of Basis Set
Superposition Errors (BSSE). Normally the bonding energy (E_bond) of a
molecule C, composed of fragments A and B, is calculated as the total
energy of C (E_C) minus that of the isolated A (E_A) and that of the
isolated B (E_B). Thus E_bond = E_C - E_A - E_B. In ADF, this can be
done in one stroke by calculating C using fragments A and B.

In the BSSE calculation, the energy of each fragment is calculated
using the basis functions of the entire complex. Thus the modified
energy of fragment A (E_Asup ***) is calculated with normal A atoms
and ghost atoms at the location of B's atoms. Likewise, the modified
energy of fragment B (E_Bsup ***) is calculated with normal B atoms
and ghost atoms at the location of A's atoms. The ghost atoms in the
calculations have their normal basis (and fit) functions. However,
they do not have a nuclear charge and do not contribute electrons to
the system. One can define a ghost atom in an ADF calculation by using
the element name Gh.atom, for example, Gh.C, see the section on ghost
atoms.

The BSSE is E_Asup *** + E_Bsup *** - E_A - E_B. The BSSE-corrected
bonding energy is E_C - E_Asup *** - E_Bsup ***.

See also:

  * Example: Basis Set Superposition Error (BSSE): Cr(CO)5 +CO

  * Tutorial: Basis set superposition error (BSSE)
