For POTEntial=SYMMetry, a soft-square function
(Eq. 20.12) 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 20.3.4), 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
(20.13)
where denotes the distance difference between
the two assigned distances.
The distances and are obtained by various
averaging methods as described in Section 20.2.