When this variable is set to a value different than 1 (the default) a combined mixing scheme will be used, with MixInterval
- 1 steps of linear mixing followed by 1 step of the selected
mixing. For the moment this variable only works with DIIS mixing.
scf/mix.F90 : 297
call parse_variable(namespace, trim(prefix)//'MixInterval', 1, smix%interval)