In some cases, Broyden mixing is too aggressive. In this case, you can switch to linear mixing with a line
linear_mixing a1 a2 a3
which mixes the potentials according to the formula:
That way, the higher wavenumbers are mixed stronger, while the tricky small G components are mixed less.
Defaults: linear_mixing 0 0.6 0.05
To switch linear mixing on, a1 has to be >0. This switches off Broyden mixing. a1=0.2 seems to be a good value to start with.