# *PROJECTION¶

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 *C*_{1} 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.

## .VECPRJ¶

For each fermion irrep, give an *Specification of orbital strings* of orbitals to analyze.

*Default:* Analyze the occupied electronic solutions.

## .VECREF¶

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.

*Example:*

```
.VECREF
4
AFH1XX
1
1
AFH2XX
1
1
AFX1XX
1
1..43
AFX2XX
1
1..43
```

## .OWNBAS¶

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.

## .ATOMS¶

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.

*Example:*

```
.ATOMS
AFHXXX
1
AFXXXX
1..43
```

## .POLREF¶

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.

## .PROTHR¶

Set threshold for absolute value of projection coefficients to be printed.

*Default:*

```
.PROTHR
0.01
```

## .WGPOP¶

Split overlap densities according to weight of contributions.