Charge Flipping Charge FlippingThe FLIPPER option in PLATON implements the Charge Flipping algorithm for ab-initio structure determination. This method was first introduced by G. Oszlanyi & A. Suto (see Acta Cryst. (2004). A60, 134-141 and Acta Cryst. (2005), A61, 147-152) as an alternative structure determination procedure for high resolution (small molecule) structures.
The required input consists of a SHELX style input file (name.ins) and a reflection file (name.hkl). Alternatively, a name.cif and a name.fcf can be supplied to test the potential of the program with a known structure. (In that case, the data is the .fcf are expanded to P1 on the basis of the Laue group of the structure in the .cif).
The program is started via the command line instruction platon name.ins or platon name.cif
A default Charge Flipping run (with 25 random phase starting points) is invoked by clicking on the FLIPPER entry on the main PLATON menu.
The same operation is invoked directly from the command line with the instruction platon -Z name.ins or platon -Z name.cif.
In the default mode, a sequence a random starting phase sets will be optimised until a stable solution is found with an R-value below 30 percent. The resulting peaklist (on the file name_flp.res) is subsequently optimized (on file name_xor.res) with PLATON/EXOR and investigated for the correct space group (result in name_sol.res). The best solution name_res.res is displayed with the PLUTON link in PLATON (allowing for the deletion and/or renaming of atoms and thus producing a starting file for refinement with SHELXL on name_res.new).
Alternatively, Charge Flipping is invoked with the keyboard instruction:
FLIP ntry nloop nsolve delta percphase Uiso
where:
ntry is the number of trial random phase starts (default = 25).
nloop is the maximum number of flip cycles (default =
250,500,1000,2000).
nsolve is the number of attempted solutions before termination
(default = 3)
delta is the fraction of the maximum density in the map below which
the sign of the density is flipped (default = 0.02).
percphasPercentage of reflections treated as weak for which
the calculated phase is shifted by 90 degrees. (default = 25)
Uiso is the value used in the expression
exp(Uiso*8*pi**2*(sintheta/lambda)**2) as a multiplication factor for
F(obs) (default = 0.02).
Example of a non-default keyboard instruction: FLIP 10 50 3 0.02 5 0.02.
Example of a name.ins file:
TITL test
CELL 0.71073 10 11 12 80 90 100
ZERR 1 0.0001 0.001 0.001 0.01 0.01 0.01
SPGR P21/c
SFAC C H O
UNIT 20 24 12
HKLF 4
For testfiles download flip.ins and flip.hkl or vitac.cif and vitac.fcf
Notes:
- The supplied space group is used only to reflect the LAUE symmetry of the data set (thus P2/m should do as well in this case). The latter is used to expand the (averaged) data to triclinic.
- The scattering type of the first atom type given on SFAC is assigned to the atoms in name_flp.res.
- The info on the UNIT line is used by the EXOR link to assign atom types to peaks.
- This routine is not optimized in terms of using the fastest available FFT routine (yet).
- The FLIPPER routine is currently experimental and under further development. Preliminary experience learns that some structures come out easily and others of similar size with great difficulty or not at all. Success or failure often depends heavily on the actual choise of the delta, percphas and Uiso parameter values.
- A special Charge Flipping run is invoked through the PATT entry on the main menu. The structure is developed from starting phases set to zero.
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