The neglect of small contributions saves the CPU time ca. Fractional phase factor, or more correctly fractional lattice vector. This command is used in relation to the symmetry operation. T-vectors are calculated with the method proposed by Mintmire et al. So far, any significant difference is not found in the results between Dunlap's original method and Mintmire's method.
Analytical evaluation is fast and exact.
However the author found that there is considerable error in the numerical evaluation even with the fine grid. At present it is better to use the numerical method in order to compensate numerical errors in the overlap matrix and XC potential evaluation. At present, geometry optimization is successful only for intra-cell structure. By default, inter-cell, i.e., lattice optimization is also turned on. Partial optimization of the atomic coordinate.