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Re: Some questions about MCQDPT theory

Alex Granovsky
gran@classic.chem.msu.su


Dear Pavel,

Sorry for some delay with my reply.

>Q1: Is there any difference between kstate(1)=1,1,1,0,1 and
kstae(1)=1,1,1,1,0 for XMCQDPT. Since Heff has the same size, i am a
little bit confused, especially after reading of the manual from
GAMESS-US. I don't understand how can we use correctly kstate array,
while situation with nstate absolutely clear (nstate == Heff)

There are two forms of input. One can specify nstate and do not set
kstate array at all. The second is to provide a kstate array and no
nstate. The first way is equivalent to second one with
kstate(1)..kstate(nstate) all set to 1.

As to the question on differences between  kstate(1)=1,1,1,0,1 and
kstae(1)=1,1,1,1,0 runs, they do differ. In the first case,
the model space is formed by the first, second, third, and fifth
CASCI root. The fourth CASCI state is computed at the very early
stage of the run and only because it is required by Davidson
diagonalization procedure to compute the fifth state. After this,
the fourth state is discarded and CASCI states are renumbered to
condense their labels. This means that the fifth CASCI state is
referred to as the CASCI state #4 throughout the rest of computations.

>Q2: Is there any correlation between nstate (SA-CASSCF) and Hef.
Thus, if Heff <= nstate then no problem at all. But if Heff > nstate
i am confused about eigenvalues of Hef and nature of "extra" states
from CAS-CI side. Could you give, please, some advice if any, about
correlation between nstate (CAS-CI) and Heff.

The states used in XMCQDPT2 and MCQDPT2 are just the CASCI states
computed with the (canonicalized) orbitals generated by the
preceeding CASSCF or SA-CASSCF computations. For instance,
one can use SA2-CASSCF to generate orbitals, then use 10 states to
construct Heff. The first two CASCI states will be those of SA2-CASSCF
roots, the extra states will be just the additional eigenvectors of
CASCI Hamiltonian.  

>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.



On Tue Oct 2 '12 6:59am, Solntsev Pasha wrote
---------------------------------------------
>Dear Alex.

>Could you clarify couple questions to me, please.

>Q1: Is there any difference between kstate(1)=1,1,1,0,1 and kstae(1)=1,1,1,1,0 for XMCQDPT. Since Heff has the same size, i am a little bit confused, especially after reading of the manual from GAMESS-US. I don't understand how can we use correctly kstate array, while situation with nstate absolutely clear (nstate == Heff)

>Q2: Is there any correlation between nstate (SA-CASSCF) and Hef. Thus, if Heff <= nstate then no problem at all. But if Heff > nstate i am confused about eigenvalues of Hef and nature of "extra" states from CAS-CI side. Could you give, please, some advice if any, about correlation between nstate (CAS-CI) and Heff.

>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.


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