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Re^4: Some questions about MCQDPT theory

Solntsev Pasha

Sorry Alex, one more thing to ask.

I have SA-CASSCF over triplets and singlets and i want to do XMCQDPT as a separate run. As you mentioned, i should use mult=1 or 3 for XMCQDPT but i wondering do i need to select all CASSCF states of the same multiplicity as requested in XMCQDPT or i should use all states (singlets and triplets) for PT2.

Thank you.


On Wed Oct 10 '12 10:12pm, Alex Granovsky wrote
>Dear Pavel,

>sorry for large delay on my side.

>>Q1. ok, it sounds very interesting. So, if an order of states from
>CAS-CI part is 1,1,1,0,1 we can setup kstate(1)=1,1,1,0,1 and
>basically remove "undesired" state from PT2 part.


>> Does it make sense?

>There are some situations when this for sure makes (some) sense.
>For instance, assume one is dealing with diatomic molecule.
>The full symmetry group is either Cinfv or Dinfh
>which is a non-abelian symmetry group. The effective Hamiltonian
>is thus block-diagonal and (provided one is interested in states of
>some particular symmetry type) one can remove other states from PT treatment.

>> Is this equivalent to setting of the AVECOE and WSTATE (XMCQDPT)
>arrays to be AVECOE(1)=1,1,1,0,1 and WSTATE(1)=1,1,1,0,1. If not, what
>is the best strategy, to use kstate or avecoe/wstate? Or, maybe, we
>can just use nstate/wstate/avecoe combination and forget about kstate.

>No you in general cannot. Both avecoe and wstate applies to renumbered
>CI states i.e. to state numbers generated after renumbering procedure.

>In general, kstate and avecoe/wstate are more or less independent.
>As to NSTATE/kstate, kstate provides the finer control.
>>Q2. What if those additional eigenvectors contribute to some low
>lying states? Do we need repeat CASSCF part once again and include
>those extra states?

>The answer depends on the magnitude of contribution. If it is large
>the answer can be "yes, it would be better to do that". As far
>as I remember this self-consistent procedure of the selection of
>the states in state-averaging was once discussed a year ago or so
>on the forum, you may find my older comments on this.
>>>>Q3: Is it good idea to do SA-CASSCF/XMCQDPT via singlets and triplets
>>>simultaneously in case of high spin-orbit coupling. Actually, i am
>>>going to check singlets only and singlets-triplets, but maybe it make
>>>no sense.

>>>I'd suggest you to use to use SA-CASSCF averaged over both singlet
>>>and triplet states to generate the same set of MOs to be used in PT2
>>>computations. Firefly v. 8.0.0 has some new features specifically
>>>tailored for these types of jobs which I'll describe in my next post
>>>to this thread.

>>Yes, sure.

>The couple of advises are:

>1. Use aldet CASCI code with PURES=.f. to generate orbitals with SA-CASSCF.

>2. Use

 $mcscf iforb=.t. $end
 $xmcqdpt iforb(1)=-1,1,1 $end 

>to generate and reuse the generated common set of canonical MOs and their energies

>3. Perform two sets of QDPT calculations, one for singlets:

 $xmcqdpt mult=1 $end 

>and second for triplets:


 $xmcqdpt mult=3 $end 

>Hope this helps.

>All the best,

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