The compatibility of different approximations to the Helmholtz free energy F is analysed in order to provide the consistent definition of the order parameters for liquid crystalline phases. In particular the diagrammatic expansion theory is compared to the weighted, also modified weighted, density approximations. The weight function w is shown to be proportional to the Mayer function. The explicit form of the weight function for the system of non-uniaxial polar molecules is given and the interaction terms of the polar and orientational order parameters in the free energy expansion are discussed.