If the keyword 'scf.Kerker.factor' is not given in your input file,
OpenMX Ver. 3.9 automatically estimates a proper value of Kerker's factor by the following
equation:

with
where
is a reciprocal vector, and
is the smallest vector
among {**b**}. The equation takes account of the dependency of on the size and anisotropy
of the system. From a series of numerical calculations it is found that the estimated value works
well in most cases.