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Re^2: CIS optimization state energy moves unreasonably

Jonas Baltrusaitis
jonas-baltrusaitis@uiowa.edu


From what I've read in the literature CASSCF should be a good thing for my molecules since it can deal with multireference character and bond breaking. I am a bit hesitant since I am not sure how to select the active space. Would TDDFT do in selecting which orbitals and electrons to correlate? Is there a rule of thumb?

And if 4 cores and 70 atoms to optimize geometry last only 2.5 days, that's really fast for (10,10)

Jonas


On Tue Jul 27 '10 2:04pm, sanya wrote
-------------------------------------
>Multifererence optimization is not too expensive if the active space is not too large. At present, I perform CASSCF optimization of a 77-atom molecule, CAS(10 orbitals, 10 electrons), ~1000 basis functions. It took about 2.5 days on a quad-core PC, 32 geometry steps. You may try, too.

>On Tue Jul 27 '10 6:05am, Jonas Baltrusaitis wrote
>--------------------------------------------------
>>I am CIS optimizing structure's 1st excited state. A very interesting behavior I am observing is that the first state calculated out of 4 (lowest energy) moves enormously in energy with the structure optimization from 290 nm to 8445 nm which is absolutely unphysical. The rest of the states stay pretty much the same

>>than coincides with two bonds adjacent to the opening up as if the structure was unstable.

>>My question is: am I optimizing to that state (is it the first one or the last one) and how do I solve this problem without resorting to a full blown multireference optimization which would be too expensive for 60 atom molecule. Calculation of course aborts at some point of bond breaking which is not surprising with TDDFT. But we have isolated these structures with bonds closed and we measured their excitation/emission so we know they are stable

>>JOnas


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