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Crystallographic Symmetry Interactions

      Crystallographic symmetry interactions (e.g., packing interactions) between the molecule(s) located in the asymmetric unit and all symmetry-related molecules surrounding the asymmetric unit are computed by

where the first sum extends over all cystallographic symmetry operators () and is defined by


is the matrix that converts orthogonal coordinates into fractional coordinates, and the second sum extends over all pairs of atoms for which is less then a specified cutoff selected by the constraints interaction statement and and are the coordinates. The function defines the minimum image distance in fractional coordinate space. It operates separately on each component of the three-dimensional vector , where the operation on each component x is given by


The function is defined as the integer part of x, and is defined as the sign of x. The nonbonding interaction energy between two atoms is independent of whether it is an intermolecular or an intramolecular interaction. The atom/group search and the update of the interaction list are carried out analogously to the intramolecular case. (Refer to Section 12.3 for the definition of symmetry operators  and the unit-cell constants  that define .)

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