  The modified quasichemical model in the quadruplet approximation (MQMQA)
considers the first- and the second-nearest-neighbor coordination and
interactions, particularly useful in describing short-range ordering in complex
liquids such as molten salts, slag in metal processing, and electrolytic
solutions. The present work implements the MQMQA into the Python based
open-source software PyCalphad for thermodynamic calculations. This endeavor
facilitates the development of MQMQA-based thermodynamic database with
uncertainty quantification (UQ) using the open-source software ESPEI. A new
database structure based on Extensible Markup Language (XML) is proposed for
ESPEI evaluation of MQMQA model parameters. Using the KF-NiF2 system as an
example, we demonstrate the successful implementation of MQMQA in PyCalphad
through thermodynamic calculations of Gibbs energy, equilibrium quadruplet
fractions, and phase diagram, as well as database development with UQ using
ESPEI. The present implementation offers an open-source capability for
performing CALPHAD modeling for complex liquids with short-range ordering using
MQMQA.
