is related by NCS to input coordinate X according to
where 
is a 3x3 rotation matrix and 
is a translation vector.
The 
matrices and the 
vectors are entered using the
XNCSrelation and NCSRelation statements. The first 3
input vectors correspond to the ROWS of 
, and
the 4th vector
is 
. It
is not necessary to input the identity transformation, since the
program sets it by default, but the user may do so for completeness.
The NCS-related interprotomer nonbonded contacts can be turned on by turning on the packing energies PVDW and PELE. Under this option, these energies are for NCS-related molecules only; the lattice energies are automatically shut off.
Many users will have the NCS relations defined in a skew frame (an auxiliary frame) in which the NCS relations have simple forms (e.g., pure rotations). The SKEW option allows a user to specify the relation between this frame and the orthogonal Å frame using a rotation matrix and translation vector. Coordinates in the skew frame are also given in Å. In this case, XNCSrelation and NCSRelation statements take the NCS relations in the skew frame. To use this option, SKEW must be given before XNCSrelation and NCSRelation.
For defining the skew frame, the algebraic
conventions of Bricogne's (1976) skewing and averaging
method
have been adopted, except
that the reference frame is the orthogonal frame
used by XPLOR. This corresponds to Bricogne's ijk frame. The matrix
is Bricogne's ``transition matrix."
The vector 
gives the coordinates of the origin of the skew frame
in the reference frame, and therefore corresponds to Bricogne's
.
Using the following conventions:
= origin of skew
frame in the reference
frame (in Bricogne's package, 
is in
fractional crystallographic coordinates; 
is the cell
orthogonalization matrix)
and 
(in
skew frame)
and 
one obtains the following relationships:
Users wishing to convert their Bricogne GENERATE mode 1 input to the XPLOR input should note the following:
is input as ROWS, not columns as in GENERATE.