I'm sorry for the late reply.
Please find my answers below intermixed with your questions.
On Wed Feb 4 '15 1:40am, olgerdovich wrote
>Nice! Actually, it works!
>Thanks a lot. And to minimize troubling you and your colleages on this board bothering with problems of this sort further, could you, please, make a couple of questions clear?
>First, what is(are) the trait(s) of my species that makes its slow convergence "more or less expected"?
There are several factors here.
First, anionic systems usually converge slower than neutral systems.
In addition, your systems are (quasi-)symmetric and you do not use
symmetry. This may worsen SCF convergence.
Finally, ROHF solution may mimic excited states, not necessary
the ground state. For instance, ROHF and UHF solutions for H2O+
cation have different symmetry and hence correspond to different
electronic states. My guess is that something similar may have
place for your systems as well. In this case, SCF tends to oscillate
between different solutions of different types or different symmetry.
>Second, what are the rules of adjusting the shift value? Since this powerful remedy secures covergence at the cost of its speed, how should one search the happy medium to get a steady convergence in reasonable time?
The default value of shift is 0.1 Hartree. This (nearly optimal)
value was selected based on numerical experimentation with different
molecules. For the badly-behaved SCF cases one may increase it to
0.2 and then to 0.5 Hartree. 0.5 Hartree is usually enough to converge
difficult SCF cases.
>You use the value of 0.5 and it works for the given case, but for isomeric system it does not work. There is a problem of oscillating of energy during geometry optimization after several steps, so that two adjacent steps differ significantly in energy while odd and even steps coverge apart rather fast, having essentially constant difference between even and odd steps all the time. Is such a case (which appears to need a damping) a subject for "fshift" remedy?
Try rstrct option in $scf.
>Finally, each successful step in geometry optimization prints "energy converged" or "density converged" or "diis converged" (or something like this). What does each of these statements point at and how one could use it struggling against convergence problems?
There are several convergence criteria used by SCF code.
The messages you mentioned are for user information only and
correspond to the criterion used to decide upon a convergence of SCF