[CP2K-user] [CP2K:17289] Spin polarized KS with Relax_multiplicity does not converge

[Laura Scalfi]

Dear Cp2k forum, 

We are trying to run a graphene sheet with a Fe defect (containing 1 Fe 
atom and 4 N atoms, 155 atoms in total), using spin polarized Kohn-Sham 
(UKS). We managed to run fixed multiplicities of 1, 3 and 5 using the 
keywords UKS and MULTIPLICITY and the orbital transformation algorithm. 
However, we would like to let the multiplicity relax using 
RELAX_MULTIPLICITY keyword and we haven’t been able to make any energy 
calculation converge yet. 

We are using DIAGONALIZATION instead of OT, with ADDED_MOS 500 and a 
Fermi-Dirac smearing with temperature 300 K. As of now we succeded to get 
convergence for DIAGONALIZATION – using the standard algorithm – (I) 
without UKS, (II) with UKS and a fixed MULTIPLICITY of 1, and (III) with 
UKS and RELAX_MULTIPLICITY starting from a multiplicity of 1 (but, however 
large the RELAX_MULTIPLICITY, we did not observe any switching of spin).

Note that, even for the multiplicity 1, the SCF loop converges on a value 
higher than one found earlier in the loop (see below), and we therefore are 
not sure whether we can trust this.

###############################################

SCF WAVEFUNCTION OPTIMIZATION


Step Update method Time Convergence Total energy Change

------------------------------------------------------------------------------

1 NoMix/Diag. 0.40E+00 1.0 0.94554597 -1011.7340994699 -1.01E+03

2 Broy./Diag. 0.40E+00 1.5 0.98949973 -984.4304450815 2.73E+01

3 Broy./Diag. 0.40E+00 1.5 1.00114938 -1018.8335907387 -3.44E+01

4 Broy./Diag. 0.40E+00 1.5 0.75169399 -1001.3444674373 1.75E+01

5 Broy./Diag. 0.40E+00 1.5 0.71210144 -1019.8272742272 -1.85E+01

6 Broy./Diag. 0.40E+00 1.4 0.91325270 -1013.4277794778 6.40E+00

*7 Broy./Diag. 0.40E+00 1.4 0.50466440 -1029.6824961219 -1.63E+01*

8 Broy./Diag. 0.40E+00 1.5 0.33674220 -1021.0650257790 8.62E+00

...

40 Broy./Diag. 0.40E+00 1.4 0.00002151 -1017.3149228922 1.08E-04

41 Broy./Diag. 0.40E+00 1.4 0.00001546 -1017.3151997206 -2.77E-04

*42 Broy./Diag. 0.40E+00 1.4 0.00000072 -1017.3150173627 1.82E-04*


*** SCF run converged in 42 steps ***

########################################################


The fixed multiplicity of 3 is very hard to converge (we only managed with 
specific parameters of Broyden mixing, after 500 iteration steps, and the 
total spin moment (Mulliken population analysis) was not consistent with 
the given multiplicity). The RELAX_MULTIPLICITY case, starting from 
multiplicity 3 or 11, never converges (we set it to 1e-5, but different 
values up to 0.5 don’t change anything).

We tried Pulay mixing but we got an error of bad conditioning, and 
Direct_p_mixing seems to worsen the convergence. We also tried using the 
method OT instead of STANDARD, with CG minimizer or DIIS (as done in a test 
case with O2), and in all cases it never converged and the OT eigensolver 
never converged at each step. 

Does anyone can suggest other parameters/methods to use or point out if 
there is a mistake in our simulations? We attach here a typical input file.

Thank you very much,

Sincerely,

Laura

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