[gmx-users] Fwd: LJ cut-offs

[Dawid das]

You might also want to read the Default section over here:
http://charmm.sunhwanj.com/html/nbonds.html

where following ''equations'' give you correspondence between CHARMM and
Gromacs keywords

rvdw-switch == ctonnb, rlist == cutnb, rvdw == ctofnb

2017-01-11 13:36 GMT+01:00 Dawid das <addiw7 at googlemail.com>:

> Hi,
>
> I believe that Mohsen here did not ask about what you are writing about
> Mark. He asks about cut-offs not the exponents
> in the LJ formula.
>
> I am no expert in that but I reviewed Andrew Leach's book on that and also
> other resources and I did not find
> any reason why lower and upper cut-off for LJ interactions cannot be  0.9
> - 1.0 nm, 1.1 - 1.3 nm, etc.
> In fact Leach gives an example of 0.9 - 1.0 nm cut-offs between which the
> shifting function is applied.
> Probable reason for using the difference of 0.2 nm is that this is more
> than the length of typical chemical
> bond, while merely 0.1 nm would not give you properly thick "skin" and
> that choice would not be too
> meaningful to meet reasons for which we use cut-off and switching/shifting
> functions (which I'm sure you know).
>
> You need to keep in mind however that each force field was optimized for
> given set of cut-offs and
> they virtually become part of that FF (just like FF formula and
> parameters), so it is strongly recommended
> not to change them!
>
> Best regards,
> Dawid Grabarek
>