Why to use a scaling factor for both vdw and electrostatics in QM/MM calculations

[binju... at gmail.com]

I saw most of QM/MM input use the scaling factor for both vdw ( VDW_SCALE14 
0.5)  and  electrostatics  ( EI_SCALE14 0.8333333).
It is not clear to me why we need to do this, since we did not us the 
scaling factor in the classical MD, and the forcefield works well in the 
classical MD.
Now, we invoke the same forcefield in QM/MM calculation, but we need to 
scale the vdw and  electrostatics ?  Thanks!
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