Fooverlaps

Fooverlaps is a 2-dimensional Numeric array that holds numerical values for the spacial overlap between basis functions. It is very similar to aooverlaps, but differs because of the way ADF performs the calculation (see below for more details). The matrix indices correspond to the fragment orbitals; see the examples listed for aonames.

Background
ADF uses symmetry-adapted fragment orbitals (SFOs) as its basis. These SFOs are generally orthonormal linear combinations of atomic orbitals. This makes it difficult to determine which individual atomic orbitals form the basis in calculations that have any symmetry. In addition, ADF allows "fragment" calculations which use the molecular orbitals of the fragments (FOs, or fragment orbitals) for building up the calculated molecular orbitals.

The difficulty in handling the basis for a molecule with symmetry and the availability of extra information in the fragment calculations makes using aooverlaps (as specified for the other formats) inappropriate, except for certain circumstances. Therefore, an extra member called fooverlaps is available for the ADF parser.

Logfile requirements
ADF: There are no required inputfile options for fooverlaps to be suppored; however, if one wishes to have SFOs map directly to atomic basis functions, there are two requirements. First, the Symmetry NOSYM option must be given to force ADF to not linearly combine atomic orbitals into SFOs. Second, fragment calculations cannot be done (for obvious reasons). Also, it is suggested that Eigval 99999 99999 be put into an Eprint block of the input file of a spin-restricted calculation so that every molecular orbital energy will be printed.