Overestimated energies of excited states in IH-FS-RCCSD

[Maksim Shundalau]

Dear DIRAC experts,

I’m trying to calculate the PECs of the low-lying states of the RbKr atomic pair for an internuclear distance of 10 A at the IH-FS-RCCSD(0,1) level of theory. So far as the excitation energy of the Kr atom is too high, the energies of the excited states of the RbKr pair at 10 A should be close enough to the energies of the excited states of the Rb atom. 

The calculated energies of the 13 lowest states (5 lowest dissociation limits) agree more or less with the NIST data for the Rb atom. The energies of the next 5 states (6th dissociation limit) are overestimated by 4000 cm^-1, but other ones are overestimated by 22000 cm^-1 or even more. 

What is wrong? It is possible to fix this? 

Thanks in advance,

Maksim.

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

Hello Maksin,

as I see from your outputs, you used ECP for your calculations. 

Just general thoughts: for more precise FSCC calculations you should use all-electron scheme (X2c) and huge basis sets. Also, FSCC may not work well for highly excited states. An alternative method for obtaining excited states is KRCI.

Best, Miro

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From: dirac...@googlegroups.com <dirac...@googlegroups.com> on behalf of Maksim Shundalau <m.shu...@gmail.com>
Sent: Friday, February 17, 2023 10:55
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Subject: [dirac-users] Overestimated energies of excited states in IH-FS-RCCSD

 

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[Kenneth Dyall]

If you want to calculate excited states accurately, you need very diffuse basis sets. The standard basis sets are not designed for calculation of Rydberg states. This would explain why the excitation energies are too high. The dyall.vXz basis sets have functions for the 5s and 5p of Rb and a less adequate representation of the d and higher Rydberg orbitals; the dyall.avXz Kr basis set has only one set of diffuse functions. You would have to extend the basis to triple or quadruple diffuse functions (depending on the zeta level of the basis set). I have been looking at basis sets for Rydberg orbitals: I have optimized some triple-zeta sets for the lowest Rydberg orbital of each symmetry for the rare gases. These are not yet published, but let me know if you are interested. 

Of course the FSCC may also be an issue, as Miro mentions. However, CC is probably a good idea for the relaxation of the Kr core when a valence electron is excited to a Rydberg orbital.

Ken.

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[Maksim Shundalau]

Dear Miroslav and Kenneth,

Thank you so much for the explanation and really useful advice. I think that now I can't use all-electron basis sets for heavy atoms, but I will try in the future. My last calculation with augmented ecp-based aug-cc-pwCVTZ basis set for the rubidium atom showed a quite good agreement for the highly excited states. 

Yours,

Maksim.

суббота, 18 февраля 2023 г. в 17:16:12 UTC+1, diracsolutions: