Fock space does not converge

[Maksim Shundalau]

Dear Dirac experts,

I’m trying to calculate PECs of the low-lying doublet states of the
LiSr molecule, which belong to the first three dissociation limits:

the first is Li(^2S)+Sr(^1S) [X^2Sigma+, 0.0 cm-1];

the second is Li(^2S)+Sr(^3P) [(2)^2Sigma+, (1)^2Pi, 14317-14898 cm-1],

the third is Li(^2P)+Sr(^1S) [(3)^2Sigma+, (2)^2Pi, 14903-14904 cm-1].


I use FS-CCSD/cc-pCVTZ level of theory (sector 0,1).



In the range of the small internuclear distances (3.0-5.0 A) solution
converges, and PECs’ energies are close to the experimental/previous
theoretical ones. But for the larger internuclear distances (7.0 A)
solution diverges. If I use an intermediate Hamiltonian, the solution
converges very slowly. I think, if I will use other IH parameters, the
solution will converge faster.



But the main problem is:

The energies of three states are too overestimated (19049-19748 cm-1).
It looks like ostensibly the third dissociation limit (which arises
from the excitation of a lithium atom) is missed, and the calculation
reproduces the second dissociation limit and some of the high-lying
ones.


I have used different basis sets and IH schemes, but obtained the
similar results.


What am I doing wrong? Is there any way to fix it?



Thanks in advance,

Maksim.

[Peterson, Kirk]

Dear Marksim,

I think it is simply because your closed-shell sector (0,0) reference state cannot dissociate correctly for the 2 dissociation asymptotes that involve two open-shell fragments.

best regards,

-Kirk

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[Ilias Miroslav, doc. RNDr., PhD.]

Hello Maxim,

I would try combination (GAS) KRMCSCF + KRCI to get proper (MCSCF))orbitals for the whole dissociation curve as well as to include enough of correlation via (GAS) KRCI .

Miro

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From: 'Peterson, Kirk' via dirac-users <dirac...@googlegroups.com>
Sent: Wednesday, August 4, 2021 18:43
To: dirac...@googlegroups.com <dirac...@googlegroups.com>
Subject: Re: [dirac-users] Fock space does not converge

 

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[Peterson, Kirk]

Miro,

 

but isn't the KRMCSF code limited to a single state?

 

-Kirk

To view this discussion on the web visit https://groups.google.com/d/msgid/dirac-users/VI1PR0702MB356691E4A9A73AD4D3B43744F7F19%40VI1PR0702MB3566.eurprd07.prod.outlook.com.

[Ilias Miroslav, doc. RNDr., PhD.]

Indeed, it is, but the following KRCI - upon good molecular spinors - can describe multiple states, http://diracprogram.org/doc/master/manual/wave_function/krci.html#ciroots  .

M

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From: 'Peterson, Kirk' via dirac-users <dirac...@googlegroups.com>

Sent: Wednesday, August 4, 2021 18:59

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[Peterson, Kirk]

Maybe, but I would be worried that as I neared dissociation my underlying spinors are going to be strongly biased towards a particular dissociation channel (one of three in the current case) of the state I chose for the KRMCSCF.  That's then asking a lot for the KRCI to make up for (particularly when you typically have to truncate the virtual space as well to make it manageable). You will certainly never recover the correct degeneracies at long range with this approach.

 

A state-averaged KRMCSCF would be really nice.  :)

 

best regards,

To view this discussion on the web visit https://groups.google.com/d/msgid/dirac-users/VI1PR0702MB3566247B252BC2DC3C01127CF7F19%40VI1PR0702MB3566.eurprd07.prod.outlook.com.

[Maksim Shundalau]

Dear Kirk and Miroslav,

Thank you so much. Firstly I will try SA-CASSCF with 10 states (7 doublets and 3 quartets) and 3 active electrons via GAMESS/Firefly in order to see what's going on. 

Yours,

Maksim. 

среда, 4 августа 2021 г. в 20:16:32 UTC+3, Kirk Peterson:

[Knecht Stefan]

Dear Kirk, dear Miro,

my few cents: I agree with Kirk that a state-specific KRMCSCF (for the ground state) will NOT be suitable in this case with an ensuing KRCI targeting an ensemble of states in the dissociation channel. 

To recover the correct degeneracies, one would probably have to run a state-specific KRMCSCF for ALL states in question + state-specific KRCI for the dynamical e-correlation 

(which would be possible to a certain extent with the existing code in DIRAC) or, as Kirk suggested, go for a state-averaged KRMCSCF in the first place. 

I have the latter on my radar and it might be something to enter DIRAC23 (certainly not earlier) in a simple-minded implementation. 

with best regards

Stefan

On 4 Aug 2021, at 19:16, 'Peterson, Kirk' via dirac-users <dirac...@googlegroups.com> wrote:

Maybe, but I would be worried that as I neared dissociation my underlying spinors are going to be strongly biased towards a particular dissociation channel (one of three in the current case) of the state I chose for the KRMCSCF.  That's then asking a lot for the KRCI to make up for (particularly when you typically have to truncate the virtual space as well to make it manageable). You will certainly never recover the correct degeneracies at long range with this approach.

 

A state-averaged KRMCSCF would be really nice.  :)

 

best regards,

-Kirk

 

From: <dirac...@googlegroups.com> on behalf of "Ilias Miroslav, doc. RNDr., PhD." <mirosla...@umb.sk>
Reply-To: "dirac...@googlegroups.com" <dirac...@googlegroups.com>
Date: Wednesday, August 4, 2021 at 10:09 AM
To: "dirac...@googlegroups.com" <dirac...@googlegroups.com>
Subject: Re: [dirac-users] Fock space does not converge

 

Indeed, it is, but the following KRCI - upon good molecular spinors - can describe multiple states,http://diracprogram.org/doc/master/manual/wave_function/krci.html#ciroots  .

To view this discussion on the web visit https://groups.google.com/d/msgid/dirac-users/2DF78BD0-786F-460A-B8D1-EBA35EDE4DC3%40wsu.edu.

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[Johann Pototschnig]

Dear Maksim,

If you start from a closed shell and add one electron you cannot get a state with an open shell on the Sr.

I would suggest to first try to compute the transitions for the atom and consider which state you can compute with a certain reference / sector of the Fock space.

An way to get different states would be to start with the anion and use the (1h,0p) sector.

You could also use EA-EOM-CC which would allow for this state, but the deviations would be large.

You might take a look at https://arxiv.org/abs/2107.11234 in order to see which state you can get with which approach and what accuracy you can expect, although is a more complicated system.

best,

Johann Pototschnig

[Maksim Shundalau]

Dear  Johann,

Thanks a lot. I will try. 
Yours,

Maksim.

четверг, 5 августа 2021 г. в 11:54:14 UTC+3, pototschn...@gmail.com:

[Maksim Shundalau]

Dear Dirac experts,

 

I’m still trying to calculate PECs of the low-lying doublet states of the LiSr molecule, which belong to the first three dissociation limits (see above).

 Now I use KRCI/cc-pCVTZ level of theory (spin-free), but I have the same problem: the energies of three states, which should belong to the third dissociation limit (Li(^2P)+Sr(^1S), 14903-14904 cm-1), are too overestimated (20037-20684 cm-1).

 Could this be related to incorrect choice of the GAS shells? Or is it something else?

 

Thanks in advance,

Maksim.

четверг, 5 августа 2021 г. в 12:15:33 UTC+3, Maksim Shundalau:

[Johann Pototschnig]

1. I would recommend running without ECP.

2. If you look at your output you have the following states:

    1S(Sr)/2S(Li) _1/2   0          -37.7795980343  0.000000000000            0.000000000000              0.000000000000      (1   )         
    1S(Sr)/2P(Li)_1/2    1          -37.7142594371  0.065338597186            1.777953621312          14340.164525753595      (1   )
    1S(Sr)/2P(Li)_3/2    2          -37.7140755141  0.065522520255            1.782958422475          14380.530973583311      (3   )
    1S(Sr)/2P(Li)_3/2    3          -37.7140755140  0.065522520295            1.782958423568          14380.530982397384      (1   )
    3P(Sr)/2S(Li)_1/2    4          -37.7131849826  0.066413051735            1.807191016035          14575.980041752462      (1   )
    3P(Sr)/2S(Li)_5/2     5          -37.7131849825  0.066413051808            1.807191018033          14575.980057864814      (5   )
    3P(Sr)/2S(Li)_5/2     6          -37.7131849822  0.066413052123            1.807191026588          14575.980126869405      (1   )
    3P(Sr)/2S(Li)_5/2     7          -37.7131849822  0.066413052147            1.807191027254          14575.980132240189      (3   )
    3P(Sr)/2S(Li)_3/2     8          -37.7131771113  0.066420923066            1.807405205841          14577.707599219888      (1   )
    3P(Sr)/2S(Li)_3/2     9          -37.7131771112  0.066420923115            1.807405207172          14577.707609959896      (3   )
   ?  10          -37.6883006635  0.091297370798            2.484327763651          20037.456792803321      (3   )
   ? 11          -37.6883006635  0.091297370799            2.484327763666          20037.456792926518      (1   )
   ? 12          -37.6853529361  0.094245098214            2.564539504631          20684.408180491184      (1   )

 For 2P(Li) the splitting of the states is overestimated, while for Sr you see none at all due to the use of ECP.

I hope this helps,

Johann

[Maksim Shundalau]

Dear Johann,

 

Thanks for your answer.

The above calculation was performed with spin-free ECP for the Sr atom especially in order to see main regularities. I think that origin of the 20037-20684 cm-1 splitting is rather splitting between Pi (first two values) and Sigma (last value) molecular terms, than atomic (Li) SOC splitting. At least, for a distance of 20 A, this splitting is absent.

 

I am attaching two more files. The first of them is a calculation with spin-orbit ECP for the Sr atom, and the second one is calculation within an all-electron basis set.

 

 

Thanks in advance,

Maksim.

понедельник, 11 октября 2021 г. в 09:40:25 UTC+3, pototschn...@gmail.com: