This study presents the theory, applicability, and merits of the new THERIAK_D add-on for the open source Theriak/Domino software package. The add-on works as an interface between Theriak and user-generated scripts, providing the opportunity to process phase equilibrium computation parameters in a programming environment (e.g., C or MATLAB®). THERIAK_D supports a wide range of features such as calculating the solid rock density or testing the stability of mineral phases along any pressure-temperature (P-T) path and P-T grid. To demonstrate applicability, an example is given in which the solid rock density of a 2-D-temperature-pressure field is calculated, portraying a simplified subduction zone. Consequently, the add-on effectively combines thermodynamics and geodynamic modeling. The carefully documented examples could be easily adapted for a broad range of applications. THERIAK_D is free, and the program, user manual, and source codes may be downloaded from http://www.min.uni-kiel.de/∼ed/theriakd/.