• The (100)-MnO and (111)-ONa terminations are the most stable under investigated chemical conditions.
• The (110) surfaces exhibit negative surface energies, indicating thermodynamic instability.
• Surface relaxation, including rumpling and reconstruction, reduces surface energy and stabilizes the surface by modifying electronic structure.
• The Wulff shape of NaMn2O4 was constructed using the Gibbs-Wulff theorem, providing equilibrium morphology predictions.