RKS calculation with Doublet Multiplicity, strange?

[Matt W]

Hi Natalie,

if your system is metallic, and you are using smearing, there is not 
necessarily any spin polarization, even if there is an odd number of 
electrons in the system. Imagine running a system with 27 Na atoms in your 
supercell, for instance. Then RKS and UKS give the same results.

Do you expect any spin polarization in your case?

Matt

On Monday, October 17, 2016 at 9:29:35 PM UTC+1, Natalie Austin wrote:
>
> Hello, 
>
> I was running a calculation (calculation 1) with an odd number of 
> electrons. The default multiplicity was set to doublet. This calculation 
> ran without the UKS tag. 
>
> I compared this calculation to one in which I included the UKS keyword in 
> the input (calculation 2). The electronic energies of each calc1 and cal 2 
> were comparable to 10E-7. 
>
> I found it strange that CP2K would run a system with an odd number of 
> electrons as RKS, is this because of the smearing?
>
> Input:
> &FORCE_EVAL
>   METHOD QS
>   &DFT
>     BASIS_SET_FILE_NAME BASIS_MOLOPT
>     POTENTIAL_FILE_NAME GTH_POTENTIALS
>     &MGRID
>       CUTOFF 500
>       REL_CUTOFF 60
>       NGRIDS 4
>     &END MGRID
>     &QS
>        EPS_DEFAULT 1.0E-14
>        MAP_CONSISTENT
>     &END QS
>   &SCF
>      SCF_GUESS ATOMIC
>      EPS_SCF 1.0E-8
>      MAX_SCF 500
>      ADDED_MOS  200
>      CHOLESKY INVERSE
>      &SMEAR  ON
>          METHOD FERMI_DIRAC
>          ELECTRONIC_TEMPERATURE [K] 300
>      &END SMEAR
>      &DIAGONALIZATION
>           ALGORITHM STANDARD
>      &END DIAGONALIZATION
>      &MIXING
>           METHOD BROYDEN_MIXING
>           ALPHA   0.1
>           BETA    1.5
>           NBROYDEN  8
>      &END MIXING
>   &END SCF
>     &XC
>       &XC_FUNCTIONAL PBE
>       &END XC_FUNCTIONAL
>     &END XC
> CHARGE -1
>     &POISSON
>       POISSON_SOLVER WAVELET
>       PERIODIC NONE
>     &END POISSON
> # UKS .TRUE.        => this is uncommented in the UKS calculation
>   &END DFT
>
>
> Calculation 1:
>  DFT| Spin restricted Kohn-Sham (RKS) calculation                         
>    RKS
>  DFT| Multiplicity                                                         
>     2
>  DFT| Number of spin states                                               
>      1
>  DFT| Charge                                                               
>    -1
>
>
> Calculation 2
>  DFT| Spin unrestricted (spin-polarized) Kohn-Sham calculation             
>   UKS
>  DFT| Multiplicity                                                         
>     2
>  DFT| Number of spin states                                               
>      2
>  DFT| Charge                                                               
>    -1
>
>
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