SCF convergence

These options to the IRC keyword increase the maximum number of points on each side of the path to 15 and the step size between points to 0.3 amu bohr (30 units of 0.1 amu bohr), where the defaults are 6 steps and 0.1 amu bohr, respectively. The SCF=QC keyword requests the quadratic convergence SCF procedure, a somewhat slower but significantly more reliable SCF procedure. 

One can increase the chances that the system remains on one surface by starting each SCF cycle at each new geometry using the converged SCF results from the previous time step. We go one step further and if there is a second previous time step, the we start the current SCF process using the KS potential that is twice the previous SCF KS potential minus the second previous SCF KS potential. (So far we have only used constant time steps.) While this method does allow practical dynamics, it does not seem to be a good idea. The problem... 

Show, by comparing the difference between the SCF total energy at ne iteration and the converged SCF total energy, that the convergence of he above SCF approach is linear or first order. 

A four-fold coordinated structure obtained by decoration of melanophlogite with carbon has been determined to be one of the two lowest energy forms of zeolite type hypothetical carbon allotropes [46]. Within a converged SCF plane wave calculation the total energy of this structure was determined to exceed that of diamond by only 0.09eV/atom. The band gap was found to be almost 4eV and only 6% lower than that of diamond utilizing the local density approximation. We find the structure to be 0.12 eV/atom less stable than diamond with a band gap which is about 4.4 eV and 68% that of diamond. 

On the other hand, this procedure is by no means guaranteed to work. In many cases, the non-aufbau guess for the initial MO occupancies may suffer variational collapse as the SCF iterations proceed, such that despite the elaborate initial guess, the final, converged SCF solution is simply the lowest energy anion state. In the calculations reported in Figure 2, " it proved possible to locate the excited Rydberg-attached state in this way, but attempts to determine the variational collapse, and the potential... 

Density Matrix-Based Quadratically Convergent SCF (D-QCSCF)... 

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