[gmx-users] Fwd: LJ cut-offs

[Dawid das]

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