Firefly and PC GAMESS-related discussion club

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Alex Granovsky
gran@classic.chem.msu.su

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.

Another point is that DFT uses numerical quadratures on grids
that do not have the exact spherical symmetry - so that sometimes
one can get multiple quasi (but not exactly) degenerate solutions
of SCF depending on what exactly are the populated atomic orbitals.  

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.htm

[ This message was edited on Sat Jan 30 '10 at 8:39pm by the author ]