This variable controls how much filter preconditioner is
applied. A value of 1.0 means no preconditioning, 0.5 is the
The default is 0.5, except for periodic systems where the
default is 0.6.
If you observe that the first eigenvectors are not converging
properly, especially for periodic systems, you should
increment this value.
The allowed range for this parameter is between 0.5 and 1.0.
For other values, the SCF may converge to wrong results.