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Re^7: SCF Convergence Problem in modelling Ag cluster

Davide Vanossi

Dear Alex,
        thank you very much for this suggestion. Surely I’ll give a try to it.
Have a nice day.


On Thu Nov 10 '11 6:06pm, Alex Granovsky wrote
>Dear Davide,

>For MCSCF, I'd suggest to try using either semi-converged
>UHF orbitals, orbitals of positively charged system or system
>of different multiplicity. This may need some orbital reordering
>directives. With Firefly, SOSCF usually converges well even for
>problematic systems. If it does not, try to perform several FOCAS
>steps before SOSCF. The simplest way is to use e.g.:

 $mcscf numfo=5 $end

>This will force five FOCAS steps before launching SOSCF.

>Best wishes,
>On Tue Nov 8 '11 11:00am, Davide Vanossi wrote
>>Dear Alex,
>>          thank you very much for your kind reply.
>>Actually I know very well the need to perform an MCSCF description for Co isolated atom. But how can I obtain, for my cluster (Ag37Co), an acceptable (converged) initial guess for using it in the next MCSCF calculation? Using fshift=0.15-0.2 didn’t help a lot.
>>Thank you again.
>>Have a nice day

>>   Davide

>>On Sun Nov 6 '11 11:41pm, Alex Granovsky wrote
>>>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,
>>>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:
>>>>>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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