See Thermodynamics of Organic Compounds in the Gas State (Volumes I and II) by M. Frenkel, G. J. Kabo, K. N. Marsh, G. N. Roganov, and R. C. Wilhoit. Published by the Thermodynamics Research Center (TRC), College Station: TX. 1994.
Clow and C¥ are equation constants, and y = T / (T + TS), where TS is a constant.