S followed by 3 items giving general equivalent positions which make crystallographic sense, producing a 3x3 rotation matrix and a 1x3 translation vector.
ASSUMPTIONS MADE:
If no S cards are present at all, the space group is assumed to be P1. The x,y,z operator is always assumed to be present.
DETAILS:
For each item one or two letters (x,y,z,X,Y or Z) will be
present.There may also be a fraction, typed as n/m where n may
be 1, 2, 3 or 5 and m may be 2, 3, 4 or 6. Letters and fractions
may be preceded by a sign, + or 
, and an initial + is
optional.
There need only be enough S cards to give the generators of the group, but redundant cards are not faulted. When SYMOP reads the S cards and has formed the whole group, it will identify and print out a set of generators.
SYMOP also checks that the rotation matrix has no zero row or column. The new operator is added to an existing list, and the resulting set of operators is augmented (if necessary) to form a group. If more than 48 (primitive) operators are found in this way an error is reported.
NOTE:
It is now also possible to specify the space group on an S GRUP card, using either its short symbol or an integer in the range 1-230, all of which refer to the listings in the International Tables for X-Ray Crstallography, Vol I, 1952. Spaces must be inserted in the symbol in the obvious places, e.g. P21/c, Ab m 2, P-3 1 m. The user is advised to check that such S cards have produced the operators he expects.
WARNING:
The maximum permitted number of operators stored is usually 24 not 48. A centre of symmetry found other than at the origin will cause the structure to be treated as non-centrosymmetric. If this means that there are finally 48 operators a specially compiled version of CCSL with space for these must be used.
EXAMPLES:
S -x, -y, -z
S 1/2+X, 1/2+Y, 1/2+Z
S y, y-x, z + 5/6
S GRUP P21 21 2
S GRUP 42
ROUTINES WHICH READ THE CARDS:
INPUTS reads one S card, and SYMOP calls INPUTS repeatedly to read all the S cards and form the group. SETFC also calls SYMOP.
The complete set of operators generated by SYMOP may be printed out by calling OPSYM(IS) where IS is 1 for output of equivalent positions and 2 for output of equivalent reflections with their relative phases.