A relation which can be used to calculate the difference between the heat capacity cp and cv for liquids at various temperatures has been established where R

T

V and α are gas constant

thermodynamic temperature

molar volume and thermal expansion coefficient respectively. A and B are two characteristic constants which can be obtained by the methods of group contribution and molecular topologic indexes. The calculated results agreed with experimental values satisfactorilly. The relation is also applicable to liquid mixtures

if simple mixing rule of A and B applies.