This paper reports estimated coefficients for the a function based on vapor pressure data to a series of 438 components, mainly hydrocarbons, for both van der Waals and Peng-Robinson equations of state (EoS). The alpha function utilized for the attractive term is the one proposed by Mathias and Copeman in 1983, and it improves the cubic EoS calculations. Accurate vapor-pressure correlations and critical data required for the estimation of the constants were retrieved from two databases, i.e., AIChE DIPPR and the Reid et al. (The Properties of Gases and Liquids; McGraw-Hill: New York, 1987) reference book. The method of calculation is described, and the constants are tested with respect of their representation of thermodynamic properties, mainly in terms of the vapor-pressure deviations. The behavior of density and enthalpy EoS predictions is also evaluated. The estimated coefficients for the a function are reported in the Supporting Information and may be applied as a data bank in a simulator, providing more accurate thermodynamic property calculations in the recommended temperature range, as demonstrated.