[CP2K-user] [CP2K:16633] Spin polarization and multiplicity in H-abstraction on a SiC surface where total magnetic moment changes

[Senja Ramakers]

Dear CP2K developers and users,

I have a question regarding spin polarized calculations and the use of the 
multiplicity and relax_multilplicity tags. 

I would like to calculate the H-abstraction energy on my Si-terminated 
4H-SiC (0001) surface. 

Initial structure: each Si surface atom has one dangling bond which is 
passivated by hydrogen atom (non magnetic groundstate, magnet moment = 0).

Final structure: one Si-H bond is broken, the H atom is moved away from the 
surface. The dangling bond and H atom now have a spin polarized ground 
state in which both unbound electrons occupy one spin channel (spin 
polarized groundstate, magnetic moment = 2).

I have plotted my results for VASP and CP2K in the attached figure. The 
nonmagnetic calculations show good overlap (blue and purple curve).

For vasp, one can fix the total magnetic moment with NUPDOWN = 0/2. If one 
sets this to 2 and starts with a ferro magnetic initial guess for all atoms 
(MAGMOM = N*1), the orange curve is retrieved. One can see that the 
groundstate is nonmagnetic until 3.4A and then becomes ferromagnetic. 
Additionally, one can not set NUPDOWN and let it relax (red and green curve 
for two different initial guesses). In this case, VASP is close to the 
correct transition but not perfect. 

In CP2K, once I turn on the magnetization with UKS and MAGNETIZATION = 1 in 
the &KIND section, the curves are all over the place and hop in and out of 
different magnetic states. I tried to tune this with the MULTIPLICITY and 
RELAX_MULTIPLICITY tag without success. 

My questions are:

1. How can I fix the total magnetic moment so that I could reproduce the 
orange VASP curve with CP2K?
2. What's the influence of the multiplicity on this calculation (I tried 
the default 1 and the triplet/3 settings). I suppose my system is similar 
to the O2 example where the Si-dangling bond and H electron are similar to 
the triplet state.
3.What does relax_multiplicity do? It seems to improve some data points 
(grey vs. brown curve) but not all.
4. Which print settings should I use to get an overview of the magnetic 
moment total (like printed in OSZICAR VASP) and on each atom (comparable to 
LORBIT = 11 in VASP) 

I would like to start using CP2K more but need to understand the spin 
polarization. Can I improve my results to come closer to VASP (or maybe 
even better)?

Thank you in advance!

Best wishes,
Senja Ramakers (PhD student at Ruhr-Universität Bochum)

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