about your question, it would be better to provide more information about your problem,
depending from problem, you may know the properties of a chemical (critical, volumetric, transport properties) and use the data regression procedures, available in many software applications, to calculate the constants to enter in chemical's database, or you may know only some limited data as molecular weight, liquid density, boiling point etc. and wish to use this limited data set for estimating the constants to enter in chemical's database, or you may wish to process petroleum fractions including hydrocarbon and non-hydrocarbon components etc. etc.
For these different problems the software which I know (Aspen, Pvtsim, Prode) sdopt different procedures and you may find the details in operating manuals...
