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Re^5: Some questions about MCQDPT theory

Alex Granovsky

Dear Pavel,

first, at present you cannot use a separate XMCQDPT2 run
for this purpose within the computation scheme I have suggested.

The reason is that with a separate runs, XMCQDPT2 code will
construct and use spin-specific (i.e., specific to singlet
or triplet sates) density matrices, canonical orbitals,
and orbital energies to be used in subsequent PT. This is
because XMCQDPT2 code works with CSFs, not determinants,
and is not capable at present to produce results averaged
over different multiplicities.

With the united MCSCF + XMCQDPT2 runs, the necessary quantities
will be computed by the deteerminant-based MCSCF code and will
be averaged over both singlet and triplet states. They later will
just be reused by the XMCQDPT2 code. The XMCQDPT2 code with

 $xmcqdpt iforb(1)=-1,1,1 $end 

ignores the wstate and avecoe arrays thus effectively inheriting
those of MCSCF. The effective state averaging for XMCQDPT2 will
then be the same as for MCSCF.

Hope this helps,

On Tue Oct 16 '12 5:48pm, Solntsev Pasha wrote
>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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