How to estimate QMMM box dimensions

Rahul Hardikar hardika... at
Thu Jan 4 06:50:20 UTC 2018

Thanks for the reply Jiang,

Maybe I should rephrase my question. I understand that GEEP algorithm 
implementation takes care of the QMMM electrostatic coupling, however that 
does not give an
idea of the box size for QM/MM region. What I am specifically referring to 
is the following section in QMMM:

CP2K_INPUT <> / 
<> / QMMM 
<> / 

where the box size of the QM/MM region can be specified with possibly 
smaller dimensions than the entire cell. So do you or other users know a 
method to estimate the minimum 
dimensions of the QM/MM cell by looking at convergence with respect to some 
observable quantity in QMMM. Any help in this direction will be helpful!

Best Regrads and belated New Year wishes to cp2k users!


On Thursday, December 21, 2017 at 4:43:20 PM UTC+5:30, Tianshu Jiang in 
Beijing wrote:
> hello Hardikar,
> The dimension of QM box depends on your system, or the quantity where you 
> are interested. 
> You can refer the paper "An Efficient Real Space Multigrid QM/MM 
> Electrostatic Coupling" written by Teodoro Laino for details.
> The realization of QMMM in CP2K is based on this paper.
> Best wishes !
> 在 2017年12月20日星期三 UTC+8下午6:06:37,Rahul Hardikar写道:
>> Dear CP2K Users/Experts,
>> Currently, in my QMMM setup I am maintaining box lengths same as the 
>> overall systems' box lengths. I am aware that one can reduce the box size 
>> for the QMMM
>> box thereby reducing the cost of QM calculations. However what is not 
>> clear to me is the role of electrostatics when QMMM box lengths are less 
>> the the overall lengths. So my question is (a) how one can estimate the 
>> QMMM box lengths and maintain the electrostatic interaction between QM and 
>> MM part consistent? and (b) how can I estimate the amount of vacuum to use 
>> to avoid any interaction between periodic images? I am using cubecruncher 
>> to crunch out plane averaged electrostatic potential and hartree energy 
>> from the corresponding cube files.  
>> Any suggestions and/or comments will be appreciated.
>> Best Regards,
>> Rahul Hardikar
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