The chemical potential of the fluid can be written as a sum of an ideal-gas contribution and an excess part: μ μ = μ μ i d + μ μ e x . The excess chemical potential is equivalent to the reversible work of inserting an additional molecule into the fluid. Note that inserting a spherical particle of radius R 0 is equivalent to creating a cavity of radius R 0 + R in the hard-sphere fluid. The SPT theory gives an approximate expression for this work W ( R 0 ) . In case of inserting a molecule ( R 0 = R ) it is