There remain a few unsolved problems when the ab initio approach was applied to study one of the most important binary shape memory alloys (SMAs), NiTi (nitinol), which is the starting material for alloying numerous multicomponent SMAs. This hinders the computational design of complex SMAs with desirable properties. In this Letter we report that including the electronic free energy in the Gibbs free energy nearly reduces by half the large errors in previously predicted martensitic transition temperatures, and the controversy on the ground state of NiTi is resolved by reaching the convergence in free-energy calculations. In addition, the present results demonstrate that the martensitic transition path in stoichiometric NiTi is directly from B2 to $\mathrm{B}{19}^{\prime }$ without intermediate phases.

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• Electronic structure
• Phase transitions
• Phonons
• Alloys
• Metals
• First-principles calculations