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

Matt W mattwa... at gmail.com
Wed Jul 12 13:30:33 UTC 2017

```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:
>
>
> 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
>
> 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 ?
>>>
>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20170712/8e93e2d6/attachment.htm>
```