This keyword activates the DIIS procedure.
ON / OFF
The DIIS procedure is switched on. This is the default.
The DIIS procedure is switched off.
The DIIS procedure is switched on after the SCF energy
error is smaller than Real.
By default the DIIS (direct inversion in the iterative subspace) procedure
 is switched on if the energy error is less than 0.01 a.u. in an
SCF step. For most cases, this results in a considerable speed-up of the
SCF convergence. However, for some systems this threshold is too large and
a SCF convergence failure will occur. In such cases, initiation of the
DIIS procedure should be manipulated with the TOL option. If the HOMO-LUMO
gap of a system is very small, DIIS can be counterproductive for the SCF
convergence. In those cases the DIIS procedure can be disabled with
the OFF option. It should be noted that in deMon2k different DIIS algorithms
are invoked for the AUXIS and BASIS options specified with the VXCTYPE keyword
(see 4.2.1). Both algorithms are based on the energy gradient with respect
to the charge-density fitting coefficients. In the case of the AUXIS option,
only charge-density fitting coefficients are used in the DIIS step. Therefore,
no extra I/O is necessary. In the case of the BASIS option, density matrices
are also used in the DIIS step and, therefore, added I/O overhead may occur.
The same holds for Fock exchange.