OMIT Instruction
OMIT s [-3] 2-theta(lim) [180]
s is a threshold for flagging reflections as 'unobserved'. Note that if no
OMIT instruction is given, ALL reflections except those with large negative
Fo^2 [i.e. Fo^2 < -3.sigma(Fo^2)] are treated as 'observed'. Unobserved data
are not used for least-squares refinement or Fourier calculations, but are
retained for the calculation of R-indices based on all data, and may also
appear (flagged with an asterisk) in the list of reflections for which Fo^2
and Fc^2 disagree significantly. Internally in the program s is halved and
applied to Fo^2, so for positive Fo^2 the test is roughly equivalent to
suppressing all reflections with Fo < s * sigma(Fo), as required for
consistency with SHELX_76. Note that s may be set to 0 (to suppress
reflections with negative Fo^2) or (as in the default setting) to a negative
threshold (to suppress very negative Fo^2) which has no equivalent in
SHELX_76. An OMIT instruction with a positive s value is NOT ALLOWED
in
combination with ACTA, because it may
introduce a bias in the final refined
parameters; individual aberrant reflections may still be suppressed using
OMIT h k l, even when ACTA is used.
2-theta(lim) defines a limiting 2-theta above which reflections are totally
ignored; they are rejected immediately on reading in. This facility may be
used to save computer time in the early stages of structure refinement, and is
also sometimes useful for macromolecules, but should not be used without very
good reason! The SHEL command may be used to flag reflections as 'unobserved'
(but retain them in the data set) above or below particular 2-theta limits.
OMIT followed by atom names but no numbers may be used to calculate an
'omit map' and is described in the section 'Atom Lists ...'.