How to calculate the HOMO-LUMO gap in a QMMM simulation?

[jts2t... at gmail.com]

<https://lh3.googleusercontent.com/-TZswRji4qTU/WWbJy5TJhdI/AAAAAAAAAAM/Ol6SXen2iSQ3e1Nfys7NOReqgdObrtJ1QCLcBGAs/s1600/QMMM%2Band%2BQM.png>
I did several simulations in the same system, which consists of 24 K atoms 
and 24 Cl atoms. The x-axis of the picture is the number of atoms in QM 
region, and the y-axis is the HOMO-LUMO gap that each simulation finally 
get. 

The tendency at the beginning seems strange. 

在 2017年7月12日星期三 UTC+8下午9:30:33，Matt W写道：
>
> Hi,
>
> I think the original paper uses four layers. I'm not sure if the MM_RADIUS 
> is the same either (in principle a fitting parameter).
>
> To me the results of the exercise look reasonable.
>
> Matt
>
> On Wednesday, July 12, 2017 at 6:27:01 AM UTC+2, jts2... at gmail.com wrote:
>>
>> Hi Matt, thanks for your reply,
>>
>> I follow the tutorial at 
>> https://www.cp2k.org/exercises:2017_ethz_mmm:qmmm, and I also look up 
>> the original paper. The question is that my consequence don't coincide with 
>> the author's simulation. The more QM atoms I use, the smaller the band gap 
>> is. The results of full QM simulation is about 4.9eV, however, the 1-QM 
>> layer/2-MM layer is about 4.7, the 2-QM layer/1-MM layer is about 4.5. 
>>
>> I did not misunderstand the QM atoms and the MM atoms. I use the QM_KIND 
>> subsection to specify the QM atoms. So, what is the problem ?
>>
>> 在 2017年7月12日星期三 UTC+8上午12:09:05，Matt W写道：
>>>
>>> Hi,
>>>
>>> your setting to get the HOMO-LUMO gap is correct. 
>>>
>>> QMMM is not straight forward to use (in any code). Check you are happy 
>>> with both the QM and the MM bit before trying to combine them.
>>>
>>> In general the gap will not be a smooth function of the number of atoms 
>>> in the QM region - too small and you won't have enough states to form a 
>>> 'band' and it is likely that the gap will be too large - in between it will 
>>> depend on the quality of the coupling between QM and MM.
>>>
>>> Matt
>>>
>>>
>>>
>>> On Tuesday, July 11, 2017 at 2:17:01 PM UTC+2, jts2... at gmail.com wrote:
>>>>
>>>> Dear researchers and developers,
>>>> I am a beginner in CP2K, and I major in computational mathematics. So 
>>>> the concepts about chemistry are new to me. I want to know how to get the 
>>>> HOMO-LUMO gap in a QMMM simulation. The setting in my input file is as 
>>>> follows
>>>>
>>>> &PRINT
>>>>          &MO_CUBES
>>>>              NHOMO -1
>>>>              NLUMO -1
>>>>              WRITE_CUBE F
>>>>          &END MO_CUBES
>>>>
>>>> which locates at the &DFT subsection. 
>>>>
>>>> But the results seems unreasonable, the result of 2-QM-atom is even 
>>>> larger than the situation if I use full QM calculation. The whole system 
>>>> consists of 48 atoms. But if I specify 12 atoms as QM-atom, the result is 
>>>> less than the result of full QM calculation. So, is there any error in the 
>>>> setting ? 
>>>>
>>>>
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