File space requirements

A typical pw.x run will require an amount of temporary disk space in the order of O double precision complex numbers:

O = N_k ^. M ^. N + q ^. Nr₁ ^. Nr₂ ^. Nr₃

where q = 2 ^. mixing (number of iterations used in self-consistency, default value = 8 ) if disk_io is set to 'high' or not specified; q = 0 if disk_io='low' or 'minimal'.