NKRED

NKRED = [integer]
Default: NKRED = 1

Description: NKRED specifies an uniform reduction factor for the q-point grid representation of the exact exchange potential and the correlation part in GW calculations.

One may restrict the sum over q in the Fock exchange potential (or one of its short range counterparts) to a subset, {q_k}, of the full (N₁×N₂×N₃) k-point set, {k}, for which the following holds

\mathbf {q_{k}} =\mathbf {b} _{1}{\frac {n_{1}C_{1}}{N_{1}}}+\mathbf {b} _{2}{\frac {n_{2}C_{2}}{N_{2}}}+\mathbf {b} _{3}{\frac {n_{3}C_{3}}{N_{3}}},\quad (n_{i}=0,..,N_{i}-1)

where b_1,2,3 are the reciprocal lattice vectors of the primitive cell, and C_i is the integer grid reduction factor along reciprocal lattice direction b_i. This leads to a reduction in the computational workload by a factor:

{\frac {1}{C_{1}C_{2}C_{3}}}

In case one sets NKRED, the grid reduction factors will be uniformly set to C₁=C₂=C₃=NKRED. If one wants to specify separate grid reduction factors for C₁, C₂, and C₃ one should use C₁=NKREDX, C₂=NKREDY, and C₃=NKREDZ, respectively.

This flag also applies to GW type calculations with a similar speedup. In GW type calculations, analogously to hybrid functional calculations the outermost loop over the momentum transfer q is reduced to a subgrid specified by the NKRED parameters.

Mind: there are circumstances under which NKRED and NKREDX,Y,Z should not be used!