It is possibile to apply restraints to an individual atom based on its
distance from a plane defined by the normal vector 
and the
atom's reference coordinate ( atom properties refx, refy, refz).
An atom for which plane restraints are defined experiences restraints
only in the direction of 
.
Plane restraints
are defined as
where the sum extends over all atoms with negative individual weights 
. The
symbols in Eq. 7.2
correspond to those in Eq. 7.1.
By default, plane restraints are turned off. To enable plane
restraints, a nonzero normal vector 
has to be specified
using the restraints harmonic statement and assigning a
negative weight 
for the
atom property HARM. Note that plane restraints are computed
only for atoms with 
; otherwise point restraints (see
Section 7.1.1) are applied, even if a nonzero normal
vector is defined. This permits the simultaneous use of point and
plane restraints, however, for disjunct sets of atoms. If
, plane restraints are disabled.
