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Re^3: orbital selection for active space. natural orbitals and cityp=aldet/guga

Alex Granovsky
gran@classic.chem.msu.su


Dear Pavel,

>I like your idea to run MCSCF-CISDT(or CISDTQ) for large active
space and check the orbitals after that. One more thing i would
like to clarify. How do you think, is it good idea to use MCSCF
calculation with excitation lower than that corresponding to
CASSCF case? Thus, if for MCSCF-FORS iexcit=6, may i use
iexcit=4 + QDPT2? Or QDPT2 was designed specifically for
MCSCF-FORS case?

With Firefly, QDPT2 was formally designed for CASSCF or CASCI
references. This does not mean you cannot use incomplete spaces
with QDPT2. For instance, QDPT2 code provides possibility to
select CSFs by their weight. I believe that any reasonable
approximation to CAS space should result in reasonably accurate
data.

All the best,
Alex



On Tue May 21 '13 6:01pm, Solntsev Pasha wrote
----------------------------------------------
>Dear Alex.
>I like your idea to run MCSCF-CISDT(or CISDTQ) for large active
space and check the orbitals after that. One more thing i would
like to clarify. How do you think, is it good idea to use MCSCF calculation with excitation lower than that corresponding to
CASSCF case? Thus, if for MCSCF-FORS iexcit=6, may i use
iexcit=4 + QDPT2? Or QDPT2 was designed specifically for
MCSCF-FORS case?
>
>
>Sincerely.

>Pavel.
>
>
>
>
>On Mon May 20 '13 1:29am, Alex Granovsky wrote
>----------------------------------------------
>>Dear Pavel,

>>Your methodology makes sense providing you are interested in
>>a ground state. If you are interested in excited states as well,
>>a better methodology would be to run CI, or, better, a several
>>iterations of state-averaged MCSCF using focas converger with
>>iexcit set to 3 i.e. to perform CISDT or MCSCF-CISDT. This would
>>allow to describe ground and single-electron excited states
>>approximately on an equal footing. Similarly, to properly catch
>>orbitals required to describe two-electron-excited states within
>>CASSCF one may use CISDTQ.

>>All the best,
>>Alex
>>
>>
>>On Sat May 18 '13 4:14am, Solntsev Pasha wrote
>>----------------------------------------------
>>>Dear friends.

>>>Please, correct me if i am wrong. I have some high symmetric molecule with transition metal. I am thinking about CASSCF for the molecule. I did DFT calculations and found all orbitals which were proposed by group-theoretical analysis and used them as starting orbitals for following calculations. The active space is too large: 18e19o. Since i am going to use cistep=guga for CASSCF i decided to run cityp=guga for my active space (iexcit=2, nstate=16, wstate=1,-0) for my large active space to estimate the population of the orbitals. I am interesting mainly in ground state, but of course low lying excited states also will be considered. After cityp=guga run the population of the natural orbital in atomic basis was analyzed. I found that population of two the lowest orbital and two the highest orbitals within my active significantly differ from the other orbitals. Does it mean i can remove two valence and two virtual orbitals from the active space and thus contract my active space 18e19o->14e15o for the casscf run? Does this methodology make sense?

>>>Sincerely,
>>>Pavel.

>>>


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