Insertion free method for determining the phase diagram of complex fluids.

We introduce a structural order parameter to determine phase diagrams in complex fluids without particle insertion or specialized simulation ensembles. The order parameter, termed the mean local affinity, is based on the local composition or density surrounding each particle and measures whether a particle resides in a locally homogeneous or heterogeneous environment. We validate the approach on three systems: the Widom-Rowlinson mixture, a symmetric polymer blend, and the Lennard-Jones fluid. In all cases, the mean local affinity exhibits finite-size scaling and yields phase diagrams consistent with established methods. The method relies only on structural information and can therefore be applied as a post-processing tool for complex macromolecular systems where conventional approaches are difficult to implement.