&optimization_variable
    STRING name = NULL;
    STRING item = NULL;
    double lower_limit = 0;
    double upper_limit = 0;
    double step_size = 1;
&end
name -- The name of the element.
item -- The parameter of the element to be varied.
lower_limit, upper_limit -- The lower and upper limits allowed for the parameter.  If these are
equal, the range of the parameter is unlimited.
step_size -- The initial step size (``simplex'' optimization ) or the grid size in this dimension (``grid'' or ``sample'' optimization).