How to estimate QMMM box dimensions

[Rahul Hardikar]

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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