The symmetry operator for the label_atom_id in the contact.
The 1_555 notation is crystallographic shorthand to describe a particular symmetry operator
(the number before the underscore) and any required translation (the three numbers following
the underscore). Symmetry operators are defined by the space group and the translations are
given for the three-unit cell axis (a, b, and c) where 5 indicates no translation and numbers
higher or lower signify the number of unit cell translations in the positive or negative direction.
For example, 4_565 indicates the use of symmetry operator 4 followed by a one-unit cell translation
in the positive b direction.
One of the atoms in the contact will have symop="1_555" that indicates the identity operator.