Usually IALLD is set to 0: IALLD=0. IALLD must be changed if you want to use the Davidson-block iteration scheme (IALLD=1) or the scheme doing a direct minimization of the Kohn-Sham total energy functional using conjugate gradient techniques (IALLD=2) (see section 7.38). Both algorithms require additional storage for another one (IALLD=1) or two (IALLD=2) sets of wave functions.