[CP2K-user] [CP2K:19992] Inquiry on ROKS Calculation Schemes within CP2K

[Jürg Hutter]

Hi

these methods / implementations are based on the work by Irmgard Frank.
You can find earlier work e.g. by Claude Daul and also M. Filatov.
There has been recently some new work by other groups that is closely related.

1) this is for single determinant only (all excess electrons in one spin channel)
2) I think it is correct, but I would have to look more closely
3) ROKS was implemented before MGGAs. It seems the ROKS branch was not updated for these type of functionals.
4) See the work of Daul and Filatov

regards
JH

________________________________________
From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Lukas Razinkovas <lukasrazinkovas at gmail.com>
Sent: Sunday, March 3, 2024 8:19 AM
To: cp2k
Subject: [CP2K:19983] Inquiry on ROKS Calculation Schemes within CP2K

Dear CP2K Community,


As a newcomer to CP2K (my experience is with VASP, GPAW, and Quantum Espresso), I have been exploring CP2K's capabilities, particularly for modeling low-spin bi- and tri-radical states of crystal defects through ROKS (Restricted Open-Shell) calculations for potential energy surface analysis. I am pleasantly surprised by this code's performance and capabilities.


However, I am slightly confused with ROKS calculations within CP2K, especially regarding the implementation and application of its varying schemes, namely:


  *   High-Spin ROKS (default ROKS_SCHEME value)
  *   GENERAL (an alternative ROKS_SCHEME value)
  *   Low-Spin ROKS


I would greatly appreciate any directed literature or resources that elaborate on these schemes' implementation and practical use.


Additionally, I have a few specific inquiries that I hope could be clarified:


  1.  Is the High-Spin ROKS scheme tailored for single-determinant maximal spin states, where all unpaired electrons align in one spin direction? Can it also model mixed (spin-contaminated) single-determinant states for the von Barth/approximate spin projection method?
  2.  For modeling the doublet state in a triradical scenario, I am considering the following configuration:
&LOW_SPIN_ROKS
  # DOBLET: E(D) = E(Q) - 3/2*E(Q) + 3/2*E(M)
  ENERGY_SCALING -1.5 1.5
  SPIN_CONFIGURATION 1 1 1 # Psi(Q) |a a a>
  SPIN_CONFIGURATION 1 1 2 # Psi(M) |a a b>
&END LOW_SPIN_ROKS
Is this the correct approach?
  3.  I encountered difficulties running LOW_SPIN_ROKS with the R2SCAN functional (however, it worked with PBE). Are there compatibility issues with meta-GGA functionals?
  4.  Can the energy outcomes from maximal-spin and low-spin calculations be compared to deduce exchange splitting (excitation) energy?


I look forward to any recommendations or advice you could provide.



Best wishes,

Lukas

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