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 
.)
