Three-step build-up of the bonding

The approach of ADF is based on fragments. This applies not only in the analysis at the end of the computation but also in the set-up of the program. The computation of the molecule from its constituent fragments takes place in three steps, and these are reflected in the analysis of bond energy components.

First, the (free, unrelaxed) fragments are placed at their positions in the molecule. This implies an electrostatic interaction: for each fragment the Coulomb interaction of its undisturbed charge density with the fields of the other fragments.

Next, the Pauli exclusion principle is applied. Even without considering self-consistency the one-electron orbitals of the combined fragments cannot represent a correct one-determinant wavefunction because the orbitals of different fragments are not orthogonal to one another. The program performs an orthonormalization of the occupied Fragment Orbitals to obtain an antisymmetrized product. This implies a change in the total molecular charge density from the sum-of-fragments to what is called the sum-of-orthogonalized-fragments. The corresponding (repulsive) energy term is evaluated separately and is called Exchange repulsion, or alternatively Pauli repulsion. The phrase orthogonal(ized) fragments, if you find it elsewhere in this manual or in the source code of adf, refers to this aspect. The sum of Pauli repulsion and electrostatic interaction is called the steric interaction.

The third phase is the relaxation to self-consistency, with of course the ensuing contributions to the bond energy.