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Re^4: SCF Convergence Problem in modelling Ag cluster

Alex Granovsky
gran@classic.chem.msu.su


Dear Davide,

actually, Firefly' DFT code has problems converging just an isolated
Co atom with e.g., mult=4. The reason is very simple. As Sanya has already
mentioned, this state actually requires MCSCF for correct description.
Indeed, there are both empty and occupied degenerated d orbitals so SCF oscillates
between solutions having different occupation numbers for different
orbitals. Increasing $scf fshift to say 0.15 or so helps. Nevertheless,
one should generally realize that DFT can be inadequate and that
there exist multiple (quasi)degenerated solutions of KS equations for this system.


As to system with Co near Ag cluster, my guess is that as Co is far
away the cluster, the convergence problems are caused by the same
reason. Try to move Co atom closer to the surface.

Best regards,
Alex



On Fri Nov 4 '11 2:04pm, Davide Vanossi wrote
---------------------------------------------
>Dear Amir,
>         I’m absolutely agree with you and, in fact,  I tried the lanl2dz from the beginning; unfortunately this introduced, on my system, a quasi linear dependence on the basis set which is not a good companion “to cure” the SCF convergence problem. For the moment I have “solved” the problem using another quantum chemistry package (NWChem) which permits to obtain a feasible SCF convergence even for the singlet multiplicity case.
>Thank you very much for your interest.
>Have a nice day.

>    Davide
>
>
>On Fri Oct 28 '11 11:56am, Amir Nasser Shamkhali wrote
>------------------------------------------------------
>>Dear Davide
>>I think that relativistic effects can be important for Ag atom. So, it's better to perform calculations with an ECP basis set which has been included relativistic effects (spin-orbit coupling) such as LANL2DZ and etc. Also these basis sets reduces the cpu time and memory considerably. You can download them form this site:
>>https://bse.pnl.gov/bse/portal
>>Please note that spin-orbit coupling reduces spin multiplicity and do not use high spin cases. The 2S+1=3 or 5 or 7 is sufficient.

>>Best wishes


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