Re: van der walls-understanding problem

From: Peter Freddolino (petefred_at_ks.uiuc.edu)
Date: Tue Aug 26 2008 - 10:08:24 CDT

Hi Rudra,
using cutoffs with the van der Waals potential is an approximation used
to make MD computationally feasible; it is, however, a good one because
the 1/r^6 dependence of this potential makes it very small beyond a
certain radius. Once you get out into the neighborhood where people
apply cutoffs (somewhere between 8 and 12 angstroms usually, depending
on who you ask) there's going to be virtually no difference between the
interaction of different atom types because they're all near 0, hence
the uniform cutoff (also note that in the switching region the
interactions will still be different for different atom pairs). Is this
what you were getting at?

Best,
Peter

Rudra Banerjee wrote:
> i have a rather silly question.
> given that vanderwalls' potential is pair potential,how can we define
> the 'cutoff' and 'switchdist' key in .conf file, which will apply same
> value of cutoff for all atom pair? my feeling is it implies all type
> of atom combination will have same cut-off. isnt it physically arguable?
>
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> Rudra
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