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We present a combined ab initio and classical molecular dynamics study of the stability and structure of the two B19′ compound twin boundaries in NiTi:(100) and (001). The role of these compound twins in suppressing the martensitic transition temperature in this material was also considered. At T= 0 K, DFT computations showed that the (001) twin is unstable to relaxation to B33. We determined from ab initio molecular dynamics simulations, however, that this twin is stabilized at finite temperature due to entropic and anharmonic effects. To our knowledge, this is the first example of finite temperature stabilization of such a defect phase. The evolution of both twin structures with temperature was also investigated, showing distortion of the lattice parameters near the boundaries. Free energy calculations obtained for the (100) twin showed a suppression of the martensitic transition temperature of up∼ 100 K …