Improving SCF convergency of DFT

As compared to HF, DFT density typically converges slowly or often fails to converge. This post describes a general strategy dealing with these situations. GAMESS has two popular SCF convergers of DIIS and SOSCF. The DIIS works well when the density matrix is a bit far from its minimum, while SOSCF is a preferred choice once the density matrix is near to it. In addition, it is generally true that HF converges much better than DFT.
Therefore, it is better to start with HF and DIIS. The switch to SOSCF optimizer needs to be activated, once the density is near to its minimum. This can be realized by

 $dft DFTTYP=B3LYP SWOFF=1e-6 sg1=.t. $end
 $scf couple=.true. alpha(1)=0.5,0.5,0.5 beta(1)=0.5,0.5,0.5
   dirscf=.t. diis=.t. soscf=.f. damp=.t. shift=.t.
   swdiis=1e-4 $end

The important keywords are

SWOFF = 1e-6
DIIS=.t.
SWDIIS = 1e-4

The SWOFF = 1e-6 ensures that HF is utilized in the SCF cycle until the density change becomes below 1x10^-6.
The DIIS=.t. sets the initial optimizer as DIIS.
The SWDIIS = 1e-4 replaces DIIS with SOSCF,  when the density change becomes below 1x10^-4.
Therefore, the DFT cycle will be performed with SOSCF in combination with the nearly optimized HF density.