Selecting structure factors in CNS is performed using a matching operation. This means that an expression is written which is matched against the properties of the structure factors currently stored in memory. Those structure factors which match the expression are selected.
Selections may be simple, selecting structure factors on the basis of resolution. However, the creation of more complex expressions requires the combination of one or more simple expressions with the operators AND, OR and NOT. The meaning of these basic operators must be understood for construction of meaningful selection expressions.
OR: The union between the two sets of structure factors selected in the two enclosing expressions.
Example:
(d < 10 or amplitude(f1) > 50)
All structure factors at a resolution limit less than 10 Å OR where array f1 has amplitude greater than 50 are selected.
AND: The intersection between the two sets of structure factors selected in the two enclosing expressions.
Example:
(d < 10 and amplitude(f1) > 50)
All structure factors at a resolution limit less than 10 Å AND where array f1 has amplitude greater than 50 are selected.
NOT: All structure factors outside the set of structure factors selected in the following expression.
Example:
(not (amplitude(f1) > 50))
All structure factors where array f1 does not have amplitude greater than 50 are selected.
The structure factor selections may use a different set of structure factors for selection and operation:
do (sigma=50*sigma) (not (amplitude(f1) > 50))
Here the selection of structure factors is based on the properties of the f1 array while the operation is performed on the sigma array.
Simple expressions are grouped using parentheses to make clear the order of selection.
Example:
((d < 10 or amplitude(f1) > 50) and l=2)
This would select any structure factors with either a resolution limit less than 10 or where array f1 has amplitude greater than 50, which also have l equal 2. This should be compared with:
(d < 10 or (amplitude(f1) > 50 and l=2))
This would select all structure factors with a resolution limit less than 10, and also any structure factors where array f1 has amplitude greater than 50 and l equal 2.
Please see the syntax manual for a full description of the structure factor selection syntax. Below are some examples which cover many of the common structure factor selection concepts.
Selection expressions can be compounded to form more complex expressions.
Example:
(amplitude(f1) > 10 and (h > 4 or l < 10))
All structure factors where f1 has an amplitude greater than 10 and Miller indices h greater than 4 or l less than 10 are selected.