First-principles study of the charge ordered phase in κ-D3(Cat-EDT-TTF/ST)2: Stability of π-electron deuterium coupled ordering in hydrogen-bonded molecular conductors

We study the electronic and structural properties of the low-temperature ordered phase of hydrogen-bonded molecular conductors, kappa-D-3(Cat-EDT-TTF)(2) and its selenium-substituted analog kappa-D-3(Cat-EDT-ST)(2), by means of first-principles density functional theory (DFT) calculations. In these compounds, the charge ordering in the pi-electron system is coupled with the ordering of the displacements in the deuteriums forming the hydrogen bond, equally shared by two oxygens in the high-temperature phase. While the structural optimization within the standard DFT method based on the generalized gradient approximation fails to reproduce the structural stability of the charge-ordered phase, we show that a hybrid functional of Heyd, Scuseria, and Ernzerhof can reproduce structural characters of the charge-ordered phase owing to the more localized nature of the wave functions. Furthermore, using the ability of the hybrid functional to predict the electronic and structural properties, we find a stable noncentrosymmetric charge-ordered phase with another pattern of deuterium ordering.