Keyword DIIS

This keyword activates the DIIS procedure.
Options:
ON / OFF

	ON		The DIIS procedure is switched on. This is the default.
	OFF		The DIIS procedure is switched off.
	TOL=$<$Real$>$		The DIIS procedure is switched on after the SCF energy error is smaller than $<$Real$>$.

Description:
By default the DIIS (direct inversion in the iterative subspace) procedure [193] 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.