FMAP Instruction

FMAP code [2] axis [#] nl [53]

The unique unit of the cell for performing the Fourier calculation is set up automatically unless specified by the user using FMAP and GRID; the value of axis must be non-zero to suppress the automatic selection. The program chooses a 53 x 53 x nl or 103 x 103 x nl grid depending on the resolution of the data. axis is 1, 2 or 3 to define the direction perpendicular to the layers. Dispersion corrections are applied (so that the resulting electron density is real) and Friedel opposites are merged after the least-squares refinement and analysis of variance but before calculating the Fourier synthesis. This will improve the map (and bring the maximum and minimum residual density closer to zero) compared with SHELX_76. In addition, since usually all the data are employed, reflections with sigma(F) relatively large compared with Fc are weighted down. This should be better than the use of an arbitrary cutoff on Fo/sigma(F). The rms fluctuation of the map relative to the mean density is also calculated; in the case of a difference map this gives an estimate of the 'noise level' and so may be used to decide whether individual peaks are significant.

If code is made negative, both positive and negative peaks are included in the list, sorted on the absolute value of the peak height. This is intended to be useful for neutron diffraction data.

code = 2: Difference electron density synthesis with coefficients
          (Fo-Fc) and phases phi(calc).
 
code = 3: Electron density synthesis with coefficients Fo and phases
          phi(calc).
 
code = 4: Electron density synthesis with coefficients (2Fo-Fc) and
          phases phi(calc).

F(000) is included in the Fourier summations for code = 3 and 4.