For POTEntial=SYMMetry, a soft-square function (Eq. 18.10) is used to restrain signed distance differences to specified values. The two distances are specified by a pair of ASSIgn statements:
The second distance of a given pair of distances is subtracted
from the first; the distance difference can have negative
values.
The functional form is similar to the soft-square
function (Section 18.3.3), except that the
switching region is symmetrically arranged around
the ideal distance difference (i.e., the program switches to the
soft asymptote for 
as well
as for 
) and 
is
defined as
where D denotes the distance difference 
between
the two assigned distances.
The distances 
and 
are obtained by various
averaging methods as described in Section 18.2.
