Electronic and magnetic properties of carbide MXenes—the role of electron correlations

Transition metal compounds are known to be tricky for ab initio calculations mainly because of the strongly localized nature of transition metal d electrons. Nonetheless, the bulk of current theoretical studies of MXenes (transition metal carbide or nitride) relies on the density functional theory using a semilocal PBE functional, whose notorious self-interaction error grossly misrepresents electronic and magnetic properties of many well-known transition metal compounds. Although several studies have adopted Hubbard-U corrections to MXenes, the lack of systematic guidelines on how to determine the U parameters has led to a cornucopia of different results. To shed some light on the reliability of different methods (different functionals or different U parameters), we performed ab initio calculations for 22 carbide MXenes (M2CT2 with M= Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and T= O, F) using density functional theory and four different methods: PBE, SCAN, HSE06, and PBE+U. In addition to trivial improvement of electronic structures of MXenes by SCAN, HSE06, and PBE+U, we found new ground-state structures for two MXenes (Hf2CF2 and Ta2CF2) and magnetic states for three MXenes (V2CO2, V2CF2, and Mo2CF2), which have not previously been reported. In addition, we found that SCAN, HSE06, and PBE+U dramatically improve the dynamical stability of V2CO2 and Mo2CF2 compared with PBE. This paper offers an overview of a broad range of MXenes with a systematic verification of various methods.