(Experimental) If set to yes, Octopus will use the Kerker preconditioner
for the mixing operator applied in the linear and Broyden mixing schemes.
When using preconditioning with the Broyden scheme, the user should increase the Mixing from its default
to between 0.5 and 0.8, for optimal results.
The implementation follows Yoshinori Shiihara et al., Modelling Simul. Mater. Sci. Eng. 16 (2008), 035004