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

jts2t... at gmail.com jts2t... at gmail.com
Wed Jul 12 04:27:00 UTC 2017

```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

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 ?

>
> 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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