Firefly and PC GAMESS-related discussion club

Learn how to ask questions correctly

Andrei V Scherbinin
andrei.scherb@gmail.com

I had some skill in calculation of RE-doped inorganic materials some time ago,
so I would say that, first of all, calculations differ... Either you are just relaxing
local environment of your dopant ion, especially in the ground state (GS), or you are
calculating vertical transition spectrum (energies, moments etc) - you use different
methods for different purposes. So your first question looks somewhat strange...
Of course, the answer is NO! But the opposite statement to that formulated in your
question is wrong too.

When no electronic excitation occurs, it is natural to use single-reference approaches
such as DFT, for instance; however, even for RE ion surrounded by ligands, its GS usually
originates from the open 4f-shell, and hence is still quasi-degenerated, just because 4f
electrons are deeply-lying and are weakly perturbed by the environment. (In other words,
crystal field splitting of the 4f levels is typically very small.) This makes the use of
any single-reference technique troublesome for the reasons mentioned by Alex.

Btw, the 4f-in-core ECP (and associated basis sets) are designed by the Stuttgart group
specifically to avoid this problem, see

http://www.theochem.uni-stuttgart.de/pseudopotentials/clickpse.en.html

and the references cited therein. If you study a process when the 4f-shell occupancy is not
changed, you may use the corresponding 4f-in-core-ECP, and a conventional DFT method, and no
quasi-degeneracy effects are expected.

On the contrary, if the occupancy of 4f-shell changes, say, under electronic transition, then
4f shell cannot be hidden in the core but should be treated explicitly, so you use a usual ECP
instead, and a appropriate multi-reference approach, such as SA-CASSCF, is required.

Even so, dynamic correlation effects that are not recovered by CASSCF, are usually very pronounced
in the RE compounds, especially when 4f-shell occupancy changes, so that quantitave decription of
electronic transitions reqires more complicated methods such as XMCQDPT, for instance. Spin-orbit
coupling effects are also crutial in case of open 4f-shell.
However, for simple qualitative purposes, SA-CASSCF usually suffices.

As to T, this is just the step time in sec.

Best wishes, Andrei


On Sat Jan 23 '10 1:34am, Dmitriy wrote
---------------------------------------
>Dear Dr Granovsky,
>Thank you very much for your detailed answer.
>Can I ask another questions?
>1. Can the conlusion be done that all DFT calculations of lanthanide compounds, that can be found in literature, are not correct in principle ?  
>2. On the other side, I am still worried with message
>SCHWARZ INEQUALITY OVERHEAD:      2775 INTEGRALS, T=        0.31
>What does T value mean ? This value was found to be significantly different (for example, 0.08 and 0.29) for the same task on different machines.

>On Fri Jan 22 '10 10:42pm, Alex Granovsky wrote
>-----------------------------------------------
>>Well, similar problems actually have been already discussed several times on this forum...

>>The atomic state (term) you are trying to model is seven-fold
>>degenerate and hence, strictly speaking, the only applicable methods
>>are those that perform averaging over all seven degenerate states,
>>e.g. SA-CASSCF. However, as you are using single-state single-reference
>>method (UDFT), it is natural to expect the existence of multiple
>>solutions of SCF equations. Both your outputs are correct and wrong
>>at the same time - they are correct in the sense that there are no
>>any errors introduced by the program and the final answer is indeed
>>the converged SCF solution..., while they are wrong in the sense that
>>these not-exactly-equal energies and different SCF solutions are just
>>artifacts of the use of the improper theoretical model.

>>As to the reasons of this instability... the CPUs are slightly
>>different so there does exist the difference in round-off errors
>>etc... The HCORE guess results in properly degenerated seven
>>f-orbitals, however, as they are degenerate, the exact one which
>>will become populated in the final solution is indeed defined by
>>these round-off errors... Just look at the converged orbitals
>>and you'll find them different in these two cases.

>>Regards,
>>Alex Granovsky
>>
>>
>>On Fri Jan 22 '10 8:48pm, Dmitriy wrote
>>---------------------------------------
>>>The situation is absolutely different: the difference between energies is significant - in third decimal place. Moreover, the conditions are identical - tasks are performed in sequential mode (but in parallel mode the same discrepancy is observed), but the discrepancy in energies arises already at the first step of SCF.  

>>>On Fri Jan 22 '10 8:01pm, Roman Zubatyuk wrote
>>>----------------------------------------------
>>>>Seems to be already discussed.
>>>>http://classic.chem.msu.su/cgi-bin/ceilidh.exe/gran/gamess/forum/?C39bb915069e1-7317-802+00.htmhttp://classic.chem.msu.su/cgi-bin/ceilidh.exe/gran/gamess/forum/?C39bb915069e1-7317-802+00.htmhttp://classic.chem.msu.su/cgi-bin/ceilidh.exe/gran/gamess/forum/?C39bb915069e1-7317-802+00.htmhttp://classic.chem.msu.su/cgi-bin/ceilidh.exe/gran/gamess/forum/?C39bb915069e1-7317-802+00.htm