Integrated absolute spin density changes significantly during geometry optimization, and is not reproduced in a subsequent SCF run (no smearing)

[Ananth Govind Rajan]

Dear all,

I am using version 2.5.1 of CP2K for geometry optimizing my system of 
interest, which is a graphene nanoribbon. I have attached the input and 
output files for two separate cases: (a) a geometry optimization 
(geo_opt.inp and geo_opt.out) and (b) an SCF run with the optimized 
geometry (scf.inp and scf.out). Two XYZ files are also attached, the 
initial unoptimized file (zigzag_init.xyz), and the final optimized 
coordinates (optimized_zigzag_initial.xyz) 

As you will see, I am performing a spin polarized (UNRESTRICTED_KOHN_SHAM) 
calculation, with the MULTIPLICITY set to 1. The RELAX_MULTIPLICITY option 
is not turned on (it is set to zero by default). I am *not* using smearing. 
As the geometry optimization proceeds, the integrated absolute spin density 
(IASD) seems to increase significantly (from ~0 to ~18). Is this expected? 
I would imagine that the IASD is related to the specified multiplicity and 
therefore should not change significantly as optimization proceeds, because 
smearing is not turned on.

In fact, after geometry optimization finished, I ran a simple SCF run to 
compute the energy of the system using the optimized coordinates. 
Surprisingly, this energy is significantly different (~ 5 eV) than the 
energy obtained at the end of the geometry optimization step, despite 
having similar input files. Moreover, the IASD is also different at the end 
of the SCF run (~ 0), compared to at the end of the GEO_OPT run (~18).

Can anyone please advise what the issue might be? Why are the results so 
different between the SCF and GEO_OPT? Is it okay that the IASD changes 
during GEO_OPT?

Please let me know if you need any more information for diagnosing the 
issue.

Thank you very much

Best regards,
Ananth Govind Rajan
PhD Candidate
Department of Chemical Engineering, MIT
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