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

[Jannen]

Dear Marcella,

Thanks so much for your response. Yes, the broken symmetry system. For the 
example, two separate H atoms, is the Integrated absolute spin density 
equal to  2, since there are two unpaired electrons if there is no spin 
contamination? Do you have any idea how is the Integrated absolute spin 
density calculated? 

I have a crystal structure that has 16 Co atoms, other atoms are C, H, O. 
The formal oxidation state of Co is 2+, so each Co is d7. When I do high 
spin calculation (all Co has 3 unpaired electrons and all are spin up), the 
Integrated absolute spin density is about 53.617 and S**2 is about 601.56 
(ideally should be 600, since 48 unpaired electrons in total). 

I am confused by the Integrated absolute spin density. Why is it 53.617.  
Does my system has strong spin contamination (S**2 is 1.56 larger than the 
ideal value)? 

I really appreciated your help!

Best regards,
Jannen 

On Sunday, June 10, 2018 at 2:54:30 AM UTC-5, Marcella Iannuzzi wrote:
>
> Dear Jannen,
>
> the electrons with antiparallel spin may occupy different orbitals, having 
> two singly occupied orbitals. 
> Example two separate H atoms with antiparallel spin.
> Regards
> Marcella
>
>
> On Sunday, June 10, 2018 at 6:33:35 AM UTC+2, Jannen wrote:
>>
>> Hello Marcella,
>>
>> Can I ask a question follow up your explanation? How could the total 
>> integrated spin density be not zero when the multiplicity is 1? 
>>
>> Thanks very much,
>> Jannen  
>>
>> On Friday, January 20, 2017 at 3:03:43 AM UTC-6, Marcella Iannuzzi wrote:
>>>
>>> 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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