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Re^3: Some questions about MCQDPT theory

Alex Granovsky

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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