Effects of nonlocal correlation functionals on electron-phonon interactions in NbC

The first-principles approach is applied to study the electronic, vibrational, and superconducting properties of B1-NbC. We used a variety of density functional theory methods that involve local density approximation (PW), generalized gradient approximation; PBE, PBEsol, and GGA functionals corrected with nonlocal correlation functionals (rVV10, vdW-DF2, vdW-DF3). Although the electronic band structures from different methods show an overwhelming agreement, the phonon dispersion curves display significant differences at certain points. The optical phonons make an appreciable part of the electron-phonon coupling constant, & lambda; (up to 23%). This contribution is found to be the largest for PW and the smallest for PBE whereas the nonlocal functionals give intermediate values. This behavior is in perfect analogy with the interaction energies for the three types of methods. The nonlocal functionals, rVV10 and vdW-DF3, fine tuned & lambda; to obtain a value in close agreement with a recently reported experimental value of & lambda; = 0.848 Yan et al. Despite the fact that variations in & lambda; due to nonlocal functionals are not very large, the effects on the superconducting transition temperatures are significant. In addition, the Coulomb screening potential, & mu; * = 0.1525 gives superconducting transition temperature in the best agreement with the experiment.