Earlier versions of EosFit (Angel, 2000a) implemented a linear variation of thermal expansion as but without the truncation of the exponential term implied by the Berman equation. Fei (1995) proposed an expansion of this expression to
(with T in Kelvin). This leads to the high-temperature volume at zero pressure given as:
With this formulation, the actual values of α0, α1 and α2 that describe a V-T curve are those at 0K, and not those at Tref, so their values are independent of Tref. It also has the advantage that the derivative is exactly
at all temperatures. The disadvantage is that the full expression predicts non-physical behaviour at low temperatures because the term in T-2 causes the value of α to diverge towards infinity as T approaches 0K. If α2 = 0 the simplified form
remains mathematically valid at all temperatures although it does not yield α = 0 at T = 0K.