There are a few ways in which this long-range behavior can be achieved, the most common is to make β β ( r ) a polynomial that is constrained to become β β ∞ ∞ at long-range: β β ( r ) = ( 1 − − y p r ref ( r ) ) ∑ ∑ i = 0 N β β β β i y q r ref ( r ) i + y p r ref ( r ) β β ∞ ∞ , y n r x ( r ) = r n − − r x n r n + r x n , where n is an integer greater than 1, which value is defined by the model chosen for the long-range potential u LR ( r ) .