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Amir Nasser Shamkhali

Dear Dave
Your idea is right. Before the CASSCF claculation, you must optimize the molecular structure. Then you may run a single point calculation on optimized structure for a population analysis of molecular orbitals. Then you should consider the population analysis of HOMO-1,....,LUMO+1 orbitals to be in correct order. For example on the basis of experimental information, if HOMO is a pi orbital, and HOMO-1 is also a pi type orbital, you may found that in population analysis the HOMO-1 orbital is not pi type orbital. Thus you should run the CASSCF calculation by changing the order of desired orbitals by IORDER or JORDER keywords.

Best Wishes

On Tue Mar 1 '11 8:34pm, dave wrote
>Dear Sir
>Thank you for helping....
>I have gone through readings and I am actually confused on reorienting the proper orbital to be populated or populating the orbital I want to study, so I thought that in doing CASSCF you should already know the orbitals involved in the transition and if it involves reorientation of the orbitals i thought it means that the orbitals being populated on the first run is not actually the one you want to study. So Sir am I just to use the HOMO-1, HOMO, LUMO, and LUMO+1 orbitals populated on the first run? or am I just to guess the orbitals to be populated since the system I am studying doesn't give idea of the transition involved?
>Hope to hear from you soon..thanks!
>On Mon Feb 28 '11 10:05pm, Amir Nasser Shamkhali wrote
>>Dear Dave
>>For CASSCF calculations you should specify the active space orbitals. Also you should determine the number of electrons in these active orbitals. As the number of active orbitals and their containing electrons increases, you need more CPU time and momory. Usually for most of the molecules only HOMO-1, HOMO, LUMO, and LUMO+1 orbitals are taken into account as active space. Thus I think that information about the all of excitations are not necessary for first calculation. However, these information previously can be determined form a TDDFT calculation with a lower computational cost.

>>On Sun Feb 27 '11 8:18am, dave wrote
>>>hi firefly

>>>i am about to study exited state systems and will use CASSCF as method, is it necessary for me to know the transitions involved in doing a CASSCF calculation? because the electronic transitions involved in the system have not been studied yet..i really need your help..thanks!

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