A theory of solutions of nonelectrolytes developed by the US scientists W. G. McMillan and J. E. Mayer in 1945. The theory shows that there is a one-to-one correspondence between the equations describing a nonideal gas and those describing dilute solutions of nonelectrolytes. In particular, they showed that there is a correspondence between the pressure of the gas and the osmotic pressure of the solution. This enables an expansion for solutions to be written, which is analogous to the virial expansion of nonideal gases with analogues of the virial coefficients. These coefficients can be calculated with the analogue of potential being the potential of mean force of N solute molecules in the pure solvent. The McMillan-Mayer theory can also be extended to distribution functions.