The current solution may be projected down onto another set of coefficients generated from the basis, e.g. one may project molecular solutions down onto atomic solutions in order to evaluate atomic contributions. The coefficients of the fragments are read simultaneously and the overlap between them is taken into account [Faegri2001] .
The projection analysis can be thought of as a Mulliken analysis based on the atomic (fragment) orbitals; it is therefore very much less sensitive to the basis set.
Normally, the fragments are calculated in the full molecular basis. This is done by zeroing charges of the remaining atoms of the molecule in the basis set input and adjusting occupation in the menu file. It is, however, possible to calculate the fragments in the own basis (a subset of the full molecular basis) using .OWNBAS. The advantage is faster calculations and conservation of atomic symmetry for atomic fragments. To avoid working with symmetry-combinations of atomic centers for the fragments, it may sometimes be advantageous to dump molecular coefficients in C1 symmetry using .ACMOUT under **GENERAL and do the analysis without symmetry. When .OWNBAS is used, one then needs to calculate each atomic type only once in its own basis in order to do the complete analysis.
For each fermion irrep, give an Specification of orbital strings of orbitals to analyze.
Default: Analyze the occupied electronic solutions.
First give number of fragments to project onto, and then for each fragment give filename of MO coefficients and the number of symmetry-independent nuclei in this fragment and for each fermion irrep, give an Specification of orbital strings of reference orbitals.
.VECREF 4 AFH1XX 1 1 AFH2XX 1 1 AFX1XX 1 1..43 AFX2XX 1 1..43
Calculate fragments in their own basis.
This keyword must be used with some care as the list of fragments is assumed to be identical to that of symmetry independent centers.
Analyze molecular orbitals in terms of atomic fragments. For each atomic type give filename of AO coefficients and an orbital string (see Specification of orbital strings) of atomic orbitals to include. DIRAC assumes that the atomic orbitals are calculated in their proper atomic basis without symmetry, that is, using the .ACMOUT keyword and the resulting DFACMO file.
.ATOMS AFHXXX 1 AFXXXX 1..43
If the polarization contribution is too big, the projection analysis looses its meaning. By activating this keyword Intrinsic Atomic Orbitals (IAOs), as formulated by Gerald Knizia, are generated, thus eliminating completely the polarization contribution.
Set threshold for absolute value of projection coefficients to be printed.
Split overlap densities according to weight of contributions.