The 1:4 scaling is an arbitrary constant that we use because it affords
better reproduction of the energy surface in the context of a fairly crude
molecular model. In recent years we have augmented the implementation to
support unique1:4 scaling factors for different interactions--Amber protein
forces fields take 83.33% strength electrostatics 50% van-der Waals, GLYCAM
uses 100% for both. I don't know exactly what GAFF does.
Last year someone was having trouble with the 1:4 term fitting in ParamFit
(or, maybe they just wanted to do 1:4 fitting but they could not in
ParamFit). In any case, mdgx has a capability for this--I never found it
to be of much use, but it can be folded into the rest of the force field
data fitting pretty easily. The person who wanted to do 1:4 fitting seems
to have gotten pretty good potential energy surfaces in the result, but
IIRC it also ended up making one of the 1:4 constants really small. There
is a tutorial but it's slated for release on the new website, which is
currently in limbo. Let me know if you want to try things with mdgx and
I'll see what I can do.
Dave
On Tue, Oct 24, 2017 at 8:47 PM, Arun Srikanth <askforarun.gmail.com> wrote:
> Hello all.
>
> I was going through the tutorial given in
>
> http://ambermd.org/tutorials/advanced/tutorial23/index.html#Introduction
>
> I plan to use this for my molecule.
> As you know the GAFF forcefield scales 1-4 bonded interactions by a certain
> factor.
> Can you please tell me how is this included in the ab initio calculations
> ?. There is no mention of this in the tutorial.
>
> Please correct me if I am mistaked I am not so familiar with QM
> calculations.
>
> Thankyou
> Arun
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Received on Wed Oct 25 2017 - 13:30:04 PDT
