Initial guess of the density matrix

[marci]

Hi

you are almost right. Actually what the keywords say in this case is
that for Cu1 1 el. 4s is taken away in spin up, while 1 4s and 2 3d
are taken in spin down. The misunderstanding comes from the fact that
cp2k is working for each spin independently, always using as a
reference the full electronic configuration, and if LSD is active a
factor 2 is applied afterwards.
Then Cu is 4s-1 3d-10. The change imposed for the bias leads to Cu1
ALPHA 4s-0 3d-10 . After the factor 2 is 4s-0 3d-5.
Cu1 BETA instead becomes 4s-0 3d-4. At the end Cu1 has one unpaired
spin-up.

It should become clearer by reading the extended output that appears
with print level MEDIUM.
Example
##########
 Spin 1

 Guess for atomic kind: Cu1

 Electronic structure
    Total number of core
electrons                                         18.00
    Total number of valence
electrons                                      10.00
    Total number of
electrons                                              28.00
    Multiplicity                                                   not
specified
    S   [  2.00  2.00  2.00]
    P   [  6.00  6.00]
    D     10.00
#########

Indeed, for this system the counting of electrons after the bias must
be readjusted, because each spin channel turns out to have one
electron too much. In the output you can read:
#######
Re-scaling the density matrix to get the right number of electrons for
spin 1
                  # Electrons              Trace(P)
Scaling factor
                           33
33.999                        0.971
######

and later on

######
Re-scaling the density matrix to get the right number of electrons for
spin 2
                  # Electrons              Trace(P)
Scaling factor
                           33
33.999                        0.971
#####

Hope this helps,
Marcella


On Feb 12, 9:27 am, Simone Piccinin <piccini... at gmail.com> wrote:
> Dear Marcella,
>
> thanks a lot for the answer.
> I'm not sure I understand how to use these feature, though.
>
> The relevant section of the cu2cl6_m1_clp6cud9.inp file in the test/QS/
> regtest-bs folder is this:
>
>     &KIND  Cu1
>       ELEMENT Cu
>       BASIS_SET  DZVP-MOLOPT-SR-GTH
>       POTENTIAL  GTH-BLYP-q11
>       &BS
>         &ALPHA
>           NEL -1   0
>           L    0   2
>           N    4   3
>         &END
>         &BETA
>           NEL -1  -2
>           L    0   2
>           N    4   3
>         &END
>       &END
>     &END
>
> Does this mean that we are removing, from the alpha channel of Cu1, 1
> electron from the 4s and in the beta channel 1 electron from the 4s
> and 2 electrons from the 3d?
> If so, Cu1 now has 4 electrons less than the neutral atom. A similar
> treatment is done for Cu2. For Cl, on the other hand, it seems that we
> are adding 2 alpha
> electrons and 2 beta electrons in the 3p orbitals. Since we have 6 Cl
> and 2 Cu atoms in the molecule, the overall charge should be 2*4-6*4 =
> -16, resulting
> in a wrong number of electrons. Is this what is done, or did I
> misunderstand something?
>
> Thanks a lot.
> Best wishes,
> Simone
>
> On 11 Feb, 22:59, marci <marc... at pci.uzh.ch> wrote:
>
>
>
> > Hi
>
> > something in the direction of a biased initial guess is already
> > present in QS.
> > The ATOMIC guess is normally based on the construction of a first
> > density matrix
> > by occupation of atomic orbitals according to the electronic
> > configuration
> > of each element.
> > However, there is the possibility to modify this procedure by
> > subtracting or adding
> > valence electrons (NEL) from specific channels (identified by N and
> > L ) of selected elements.
> > This can be done independently for spin up (ALPHA) and spin down
> > (BETA).
> > Have a look at the reference inputs in cp2k/tests/QS/regtest-bs.
> > This might provide a better initial guess, also for charged systems.
> > As a safety check, if the resulting density matrix amounts to the
> > wrong number of  electrons,
> > it is rescaled by  a proper factor, and the calculation proceeds.
> > This trick is only used for the initial guess, no further constraints
> > are applied afterwards.
> > Hence, there is no guarantee that the selected configuration is kept
> > during the SCF.
>
> > Kind regards
> > Marcella
>
> > On Feb 11, 9:12 pm, Simone Piccinin <piccini... at gmail.com> wrote:
>
> > > Dear CP2K users,
>
> > > I'm studying a large molecule containing 4 Ru atoms. These are
> > > arranged as to form two pairs. I'd like to study the molecule in a
> > > singlet state, and see if the coupling within each pair is
> > > ferromagnetic (FM) or antiferromagnetic (AF). Is there a way in
> > > quickstep to guide the scf cycle toward the FM or AF solution, for
> > > example through an appropriate initial guess of the density matrix?
>
> > > Thanks for your help.
>
> > > Best wishes,
>
> > > Simone Piccinin
> > > CNR-IOM Democritos (Trieste)