Each of the terms in this formula has a theoretical basis. The coefficients formula_17, formula_18, formula_19, formula_20, and formula_21 are determined empirically; while they may be derived from experiment, they are typically derived from least squares fit to contemporary data. While typically expressed by its basic five terms, further terms exist to explain additional phenomena. Akin to how changing a polynomial fit will change its coefficients, the interplay between these coefficients as new phenomena are introduced is complex; some terms influence each other, whereas the formula_21 term is largely independent.