Preparation of diffraction data

We will assume that the diffraction data has been converted to CNS format using to_cns or CCP4. There will then be a reflection file containing the observed structure factor amplitudes and estimated errors (sigma values). Cross validation will be used to monitor the subsequent refinement of the model and also to reduce bias in map calculations. The cross validation information is added to the reflection file using the CNS task file make_cv.inp.

      cns_solve < make_cv.inp > make_cv.out [8 seconds]

A new reflection array is created, by default the name TEST is used. The array contains integer numbers, by default 1 signifies that the reflection will be used for cross-validation (i.e. excluded from refinement). The output of the make_cv.inp task file is a CNS formatted reflection file. This will contain a header which describes the data items in the file, followed by the data items themselves. The file is plain ASCII and can be viewed with any text viewer/editor. Example header from a CNS reflection file with cross validation information:

 NREFlection=     37941
 ANOMalous=TRUE { equiv. to HERMitian=FALSe}
 DECLare NAME=FOBS                   DOMAin=RECIprocal   TYPE=REAL END
 DECLare NAME=SIGMA                  DOMAin=RECIprocal   TYPE=REAL END
 DECLare NAME=TEST                   DOMAin=RECIprocal   TYPE=INTE END
 INDE     0    4    0 FOBS=   370.040 SIGMA=    28.370 TEST=         0
 INDE     0    6    0 FOBS=    52.710 SIGMA=     2.210 TEST=         0
 INDE     0    8    0 FOBS=   341.050 SIGMA=    18.230 TEST=         1
 INDE     0   10    0 FOBS=   157.510 SIGMA=     8.560 TEST=         0
 INDE     0   12    0 FOBS=   375.450 SIGMA=    22.210 TEST=         0