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Re^3: The energy shifts in XMCQDPT calculation.

Alex Granovsky


why are you using effective Hamiltonians of such a large dimension?
This does make sense quite often, however when you
perform scan along reaction coordinate which results in
geometries of dramatically different electronic structure,
you can encounter a kind of "intruder states" in your model
space. This is not the states normally called intruders but
the idea is similar. Namely, to get continuous energies one
needs to use the model space which is slowly varying (as a
subspace) along reaction coordinate i.e. which is continuous
as well. Ideally, one needs to construct the correlation diagrams
for each CASCI state one plans to include into model space,
then checks that the model space itself is continuous.
It can be difficult to achieve for large dimensions of the model
space. For small active spaces, while this is rather trivial and
there is typically no intruders of that kind.

Alex Granovsky

On Thu Aug 16 '12 6:05pm, Serzhan wrote
>On Thu Aug 16 '12 2:13pm, sanya wrote
>>The problems in QDPT frequently arise from errors or problems in CASSCF. From your outputs, I see that root flipping occurs: in 136CASSCF-10-10PT34 State 1 was closed-shell (all occupancies are close to 2 and 0), while in 135CASSCF-10-10PT34 I see fractional occupancies of orbital 5 (1.43) and 6 (0.57). Can you show us your CASSCF inputs and outputs?
>>In addition, you'd better set $XMCQDPT group as follows:

>>! (instead of kstate(1))
>>  nstate=34
>>! (additional parameters: you need 3 states, so 3 1's)
>> wstate(1)=1,1,1,-0 avecoe(1)=1,1,1,-0
>>  edshft=0.02
>> $end

>>I think this improvement alone will not help you, unless you find out what is wrong with your CASSCF.

>Thank you for your reply.

>I attached the CASSCF inputs and outputs with this post. It seems the MOs in active space remain intact, while the CI roots differ at points of interest. So that can be the cause of the jump. However, the root flipping, you've mentioned, is due to the physics of the problem. So It will be great if the energy profile could be kept smooth, while root flipping is present.

>Thanks for your help,

[ This message was edited on Thu Aug 16 '12 at 10:04pm by the author ]

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