[CP2K-user] [CP2K:12209] Re: Spin Polaization in Kohn-Sham calculations
Ron Cohen
reco... at gmail.com
Thu Sep 12 11:04:52 UTC 2019
Yes, but unfortunately it seems the results are wrong. The spin-polarized kohn-sham equations are not solved using UKS and RELAX_MULTIPLICITY. For an insulator the integrated spin must be an integer, but it is not in cp2k. The results obtained with quantum espresso for the same system are as expected. It is an insulator with integrated spin exactly 2.0, but cp2k gives 2.04. If the ks equations are not being solved, it is not a dft calculation, and the solution is not the minimum energy solution.
Sent from my iPhone
> On Sep 12, 2019, at 06:45, Vladimir Rybkin <rybk... at gmail.com> wrote:
>
> Dear Ron,
>
> regarding the first point. CP2K does spin polarization in molecular quantum chemistry style: either with spin-unrestricted KS (UKS, or LSD) or with restricted open-shell (ROKS). The first option assumes different (spatial) orbitals for different spins: orbitals for alpha- and beta-channels are found independently and are in general different. That's why Sz and S^2 are no more good quantum numbers. Integrated absolute spin density is non-integer. In the ROKS scheme, spatial orbitals are fixed the same so that the spin symmetry does not break, but some variational flexibility is lost.
>
> The RELAX_MULTIPLICITY keyword means, that CP2K should find the electronic state according to the aufbau principle. It does exactly this. The MULTIPLICITY 1 message is perhaps a "bug" of the output. It prints the default multiplicity (1) rather than the one that it will going to find according to aufbau.
>
> Yours,
>
> Vladimir
>
>
> среда, 11 сентября 2019 г., 17:37:40 UTC+2 пользователь Ronald Cohen написал:
>>
>> I do not understand the way CP2K does spin-polarized Kohn-Sham. What ought to be done in a crystal
>> is to have spin up and down channels (called alpha and beta in cp2k), solve them in their separate LSDA (or GGA) potentials for each k-point,
>> find a common Fermi level by integrating the charge to the correct number of electrons, compute the new charge and spin densities,
>> compute the new spin dependent potential, and iterate. In an insulator one should obtain an integer integrated spin, since each spin polarized band holds 1
>> electron. However, in cp2k for insulating O2 (in a cage) , for example, I find a converged integrated spin density of
>>
>> Integrated absolute spin density : 2.0381878543
>>
>> instead of 2.
>>
>>
>> In cp2k, one enters a MULTIPLICITY and RELAX_MULTIPLICITY, and I find disconcerting messages like:
>>
>> Spin 1
>> Re-scaling the density matrix to get the right number of electrons for spin 1
>> # Electrons Trace(P) Scaling factor
>> 39 40.000 0.975
>>
>> Spin 2
>> Re-scaling the density matrix to get the right number of electrons for spin 2
>> # Electrons Trace(P) Scaling factor
>> 37 36.000 1.028
>>
>> There should be nothing like this.
>>
>> If I do not enter the MULTIPLICITY key word I do not see this message,
>> but instead see:
>>
>> Multiplicity 1
>>
>> and the resulting integrated spin is similar but not identical:
>>
>> Integrated absolute spin density : 2.0411999533
>>
>> Any clarification would be greatly appreciated. I cannot find an explanation in the docs.
>>
>> Sincerely,
>>
>> Ron
>>
>
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