SCF

The SCF procedure is regulated with keys that set the maximum number of iterations, the convergence criterion, and various items that control the iterative update method. Molecules may display wildly different SCF-iteration behavior, ranging from easy and rapid convergence to troublesome oscillations. We expect that the default settings take care of most cases, but one should realize that this is a difficult and tricky subject. The user has a few (main) options to adapt the procedure to the situation at hand: simple damping or the DIIS procedure (Direct Inversion in the Iterative Subspace). Either of them can be combined with Level-Shifting.

At each cycle the density is computed as a summation of occupied orbitals (squared); the new density defines the new potential from which the orbitals are re-computed, et cetera, until convergence is reached. To speed-up convergence and to avoid non-convergent oscillatory behavior the values at the next iteration are constructed as a mixture of the computed new data and those used at the cycles before. This may involve only the previous cycle and is then called damping. Alternatively the DIIS procedure can be invoked, which is a generalization of damping to include more previous iterations.

In ADF2009.01 two methods, called ARH and Energy-DIIS, have been implemented that can solve problematic cases for SCF convergence. These methods will be discussed separately, after the main options of the SCF key. Both methods require the total energy to be calculated at each step, which makes them much more expensive compared to the standard SCF procedure, and not applicable in all cases. Therefore, these methods should only be used when the standard SCF procedure fails.