[gmx-users] cutt offs

[Ilya Chorny]

The reason I was asking was not to debate the subject. I am growing a LJ
cavity in water. When the cavity gets really big beyond the standard LJ
cutoff I run into problems as one might imagine. I was looking for a way to
overcome this problem.

By the way. A dipole-dipole interaction falls of faster than a chrage-charge
or charge-dipole.

Thanks for your help!!!!

Ilya


-----Original Message-----
From: gmx-users-admin at gromacs.org [mailto:gmx-users-admin at gromacs.org]On
Behalf Of Eric Jakobsson
Sent: Monday, December 29, 2003 4:01 PM
To: gmx-users at gromacs.org
Subject: RE: [gmx-users] cutt offs


Absolutely not.

Probably too big a gap in perception here to profitably continue such a
debate.

At 01:20 PM 12/29/2003 -0800, you wrote:
>Your example is extreme. What about the case of a single charge in
>water. The water-water Coulomb cut off should be less than the charge
>water Coulomb cutoff.
>
>Ilya
>
>
>-----Original Message-----
>From: gmx-users-admin at gromacs.org [mailto:gmx-users-admin at gromacs.org]
>On Behalf Of David van der Spoel
>Sent: Monday, December 29, 2003 12:40 PM
>To: gmx-users at gromacs.org
>Subject: RE: [gmx-users] cutt offs
>
>On Mon, 29 Dec 2003, Ilya Chorny wrote:
>
> >Its not obvious to me why the thermodynamics would get screwed up. Its
> >intuitively the same as having different cut offs for LJ vs Coul.
>That is an approximation, and since LJ potential goes to zero faster it
>can be justified (although that does not mean it is correct).
>Treating different parts of the system with different
>cut-off is worse. Let's say we divided a 6x6x6 nm system in two pieces,
>that with X coordinate < 3 nm and that with X > 3 nm. Now if we use a
>different cut-off for the first half of the system than for the other
>half, we create artifacts on the border between the two, and what's
>more,
>I suspect that for such a system in "equilibrium" the chemical potential
>
>is not the same in both halves. It will hence be impossible to define
>what
>kind of ensemble is modeled. Treating cut-offs differently for the
>protein
>and water is the same kind of division, comparable in nature to
>simulations in implicit solvent.
>
> >
> >>You would not want to do that anyway--it would really put you in
> >>thermodynamic neverland.
> >I agree, but IIRC this is what Amber does quite frequently, no cut-off
> >within the protein, and short cut-off for protein-water and water-water
>
> >(e.g. the 1 us folding sim of Duan and Kollman).
>
>And to come back to that simulation, they also determined solvation free
>
>energies from that, but from just the protein coordinates, dismiising
>all the
>solvent.
>
>Just my (possibly biased) opinion...
>
>--
>David.
>________________________________________________________________________
>David van der Spoel, PhD, Assist. Prof., Molecular Biophysics group,
>Dept. of Cell and Molecular Biology, Uppsala University.
>Husargatan 3, Box 596,          75124 Uppsala, Sweden
>phone:  46 18 471 4205          fax: 46 18 511 755
>spoel at xray.bmc.uu.se    spoel at gromacs.org   http://xray.bmc.uu.se/~spoel
>++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
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---------------------------------
Eric Jakobsson, Ph.D.
Professor, Department of Molecular and Integrative Physiology, and of
Biochemistry
Senior Research Scientist, National Center for Supercomputing Applications
Professor, Beckman Institute for Advanced Science and Technology
4021 Beckman Institute, mc251
405 N. Mathews Avenue
University of Illinois, Urbana, IL 61801
ph. 217-244-2896       fax 217-244-2909

(Currently on leave to NIH to serve as Director of Center for
Bioinformatics and Computational Biology at the National Institute of
General Medical Sciences and Chair of the NIH Biomedical Information
Science and Technology Initiative Consortium.  Usual schedule is four days
a week at NIH and three days a week at Illlinois.)



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