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

[Marcella Iannuzzi]

Dear Ananth,

the total integrated spin density can be different from zero, even if the 
multiplicity is stil one, this should not be a problem. 
After the geometry optimisation, did you try to restart also the wave 
function in addition to the coordinates?
I have noticed that in your scf.inp the SCF_GUESS is set to ATOMIC.
My guess is that the scf ends to be trapped in a metastable state ,due to 
the bad starting guess.

Kind regards,
Marcella 

On Monday, January 16, 2017 at 2:41:56 AM UTC+1, Ananth Govind Rajan wrote:
>
> 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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