Last modified: 8-feb-98

FOURIER commands perform Fourier Transformations to calculate structure factors (FORWARD) or maps of electron density (BACKWARD). FORWARD is the default.

Releted commands are **"REFLECTION"** (reflect.html) and **"MAKE MAP**
(make.html).

> FOURIER MAP inte ATOMS SELECT ... END REFLECTION SELECT ... END FFT DIRECT FORWARDS BACKWARDS RESOLUTION real real B_ADD real EXP_TABL ON OFF

Structure factors will be calculated. Either from atoms directly or by FFT-ing a map.

The forward transform automatically extracts reflections into the FCALCULATE data array. Data are not premultiplied with a resolution dependent temperature factor. If you want to scale your data, you need to initiate a reflection R-VALUE calculation followed by a REFLECTION SET AMPLITUDE command.

An electron density map will be calculated. A MAP must be specified.

In order to transform a map BACKWARDS (calculate a map from structure factors), you need to FILL the map first with the REFLECTIONS FILL command. Only FWORKSET array fills a map.

... RESOLUTION real real

Redefines the resolution range in which an FFT of a map is performed.

FFT method will be applied on a map to either transform a map into structure factors (FORWARDS) or a reciprocal space map into electron density map (BACKWARDS).

The DIRECT summation method will be applied.
Currently only the FORWARD transformation
(Fcalc from ATOMS or a MAP) can be applied.
You have to specify either **"ATOMS"** selection or a **"MAP"**.

> fourier refl sele ... end direct forw atom sele ... end > fourier refl sele ... end direct forw map inte

... MAP inte

The inte defines the map used in a fourier transformation.

... REFLECTION SELECT ... END

Selects the reflections to be used in the DIRECT calculations.

... ATOMS SELECT ... END

Selects the atoms to be used in the DIRECT calculations.

B_ADD is the overall B, which is added to individual atomic B-factors to reduce the truncation error in FFT based struture factor calculations.

Turn ON and OFF the usage of exponetial tables for generation of electron density maps (using gaussian form of atomic structure factors) and in corresponding energy terms.

> make map .. atom dens > energ dens convolution > energ x-targ on

Use of tabulated exponetials substantially accelerates electron density
map generation. **"ON"** is the default. If your machine is not really fast
don't use the DENSITY CONVOLUTION term.