Basis Set Superposition Error (BSSE)

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)