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

[binju... at gmail.com]

Now, I got some clue from Amber manual, By default, vdW 1-4 interactions 
are scaled down by a factor of 2.0, and the electrostatic 1-4 terms by a 
factor of 1.2. 

在 2018年6月6日星期三 UTC+8下午5:22:23，binj... at gmail.com写道：
>
> Dear Dawid, Thanks for you answer, I am using the standard Amber 
> forcefield. 
>
> best
>
> binju 
>
> 在 2018年6月6日星期三 UTC+8下午5:15:03，Dawid das写道：
>
>> Hello,
>>
>> Whether you scale the 1-4 interctions or not depends on the force field 
>> and system (lipid, protein, nucleic 
>> acid). For instance, I use CHARMM22 force field and I do not scale the 
>> 1-4 interactions (VDW_SCALE14 and EI_SCALE14 set to 1.0).
>>
>> You need to read how your force field was optimized and what scaling 
>> factor (if any) is suggested.
>>
>> Best wishes,
>> Dawid
>>
>> W dniu środa, 6 czerwca 2018 11:09:41 UTC+2 użytkownik binj... at gmail.com 
>> napisał:
>>>
>>> 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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