[CP2K-user] [CP2K:19520] Re: Problem with the calculation of the CUBE file and Wannier Centres using the ROKS method?

[Krack Matthias]

Hi Ilya

Thanks for contributing and fixing the print issue.

Indeed, ROKS produces only one set of orbitals, since there is also only one Hamiltonian, i.e. Kohn-Sham matrix, but different orbital occupations are applied for spin-up (alpha) and spin-down (beta) electrons which results in an alpha and beta electronic density matrix.

The LOW-SPIN ROKS implementation in CP2K based on OT plus ROTATION is experimental as indicated by a warning printed in the output. That implementation has not been maintained since a long time. Therefore, I am wondering, if the implementation works properly, e.g. for the new test case O2_mo_cubes.inp. I tried tighter thresholds and a larger cutoff and box size, but the SCF did not converge. What’s your experience and how do the results compare to ROKS in CPMD?

Best

Matthias

From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Ilya Fedorov <ilyafedorov19 at gmail.com>
Date: Thursday, 16 November 2023 at 10:59
To: cp2k <cp2k at googlegroups.com>
Subject: [CP2K:19518] Re: Problem with the calculation of the CUBE file and Wannier Centres using the ROKS method?
I solved the problem and it was accepted into the code.
1.       As I understand it, for ROKS the UKS base is used, and different spins of the same orbital have the same density. This allows to rely on the UKS codes, while you can use MO_CUBES only for spin-1. (regtest: QS/regtest-lsroks/O2_mo_cubes.inp)
2.       I added such an implementation in the code. Only two method supports:
o    NONE: No rotation, just print the orbital centers.
o    JACOBI: Only doubly occupied orbitals rotate; for singly occupied orbitals, no rotation occurs (like the NONE method). (regtest: QS/regtest-lsroks/O2_loc_wan_jac.inp)
It is possible to use only NONE and JACOBI. Other methods can be added in the future, but ROKS calculates quite slowly, and speeding up the Wannier calculation does not make a big contribution here.

Commits: https://github.com/cp2k/cp2k/pull/3108
четверг, 24 августа 2023 г. в 15:19:45 UTC+3, Ilya Fedorov:

Dear colleagues,

 Could you please help me to understand the CP2K implementation of ROKS and the Wannier Centers.

I am facing the following two problems. I can't find an answer in the documentation. And, unfortunately, I don't really understand in the code.

1) The ROKS implementation in CP2K uses a spin-restricted calculation with multiple density matrices. As I understand it, this leads to some problems with printing CUBE files (for example, for each orbital it prints two cube files for spin_1 and spin_2). Which one is correct? Maybe the sum of them gives a correct density?

(in log it print “Unclear how we define MOs in the restricted case ... skipping”)



2) I’m trying to use Wannier Centres with ROKS, but it doesn’t work in CP2K. As I understand this is also a multiple density matrix problem. I need Wannier Centres only (Many-body Position operator, no Jacobi rotation). Right now I use Wannier + ROKS in CPMD, and in CPMD the rotation is just switched off for the last two orbitals (diag(1,1)).

Best regards

Ilya
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