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Re: [ESPResSo-users] Tabulated angle potential in Espresso

From: Javier Ramos
Subject: Re: [ESPResSo-users] Tabulated angle potential in Espresso
Date: Tue, 03 Jan 2012 10:43:12 +0100
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110921 Thunderbird/3.1.15

Hi and Happy new year to everybody,

First of all, thank you very much for your previous answers.

I understood from the Espresso manual that the force is, as you say, U'(r)/r being the r the distance between two beads in the case of tabulated non-bonded potentials. However, in the case of tabulated angle potentials I am not be able to understand the meaning of r. Which is the r that I have to use?. Is the connecting distance between 1-2 beads? or 1-3 beads??? (for the angle between (1)-(2)-(3) beads)
Best regards,


On Thursday 15 December 2011 10:40:15 Javier Ramos wrote:
> Dear Espresso users,
> I am trying to use a tabulated angle potential (variant 2 in the section
> 5.3.7 of the user guide). I have two questions.
>      i) Does the angle potetial zero corresponds to the polymer is this
> position: (p1)-(p2)-(p3)?

Yes, if you mean the stretched position, where all particles are in a line.

>     ii) It is not clear for me which is the scaled length that I need to
> use for the forces. Following the explanation in the non-bonded
> tabulated potential I am assuming that the second column has to be built
> as F(theta) = - Vprime(theta) / theta where Vprime is the derivate of
> V(theta) respect to theta and theta is the angle between p1,p2 and p3
> particles. is that right?

According to the User's guide, no. It says only that the forces are scaled 
with the inverse length of the connecting vectors, which is what is required 
due to the angle. What you therefore give is the magnitude of the force on one 
of the two outer particles, multiplied by the distance of the particle to the 
center particle.


JP Dr. Axel Arnold
ICP, Universität Stuttgart
Pfaffenwaldring 27
70569 Stuttgart, Germany
Email: address@hidden
Tel: +49 711 685 67609

Dr. Javier Ramos
Molecular Structure and Properties of Polymers
Department of Macromolecular Physics
Instituto de Estructura de la Materia (CSIC)
C/ Serrano 113 bis 28006 Madrid (Spain)
Voice +34 91 561 68 00 (943112)
Fax +34 91 585 54 13

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