Supplementary Material

It concludes computational details; the origin of negative Poisson's ratios; phonon spectra; AIMD simulations; Young's modulus and Poisson's ratio; the effect of the Hubbard U parameter applied to Nb atoms on the crystal structure; PDOS and HSE06-level band structure; angular-dependent MAE and temperature-dependent specific heat under various strains; and the electronic polarization at strains of −6%, −8%, and 8%