VMD-L Mailing List

From: Axel Kohlmeyer (akohlmey_at_gmail.com)
Date: Thu Sep 08 2011 - 19:51:54 CDT

On Thu, Sep 8, 2011 at 6:38 PM, kirtana S <skirtana4_at_gmail.com> wrote:
> I want to know where I am going wrong here
>  using the script as below
>
> set sum 0
> foreach mass [$selection get mass] {
> set sum [expr $sum + $mass]
> }
> puts "mass is sum $sum"
> set com 0
> foreach coord [$sel get {x}] {
> set com [vecadd $com [$vecscale $sum $coord]]
> }
> puts "x is $com"
>
> though this runs my value for center of mass coordinate for x is a bit weird
> as my output says
> x is -17586.98
>
> where am i going wrong here

your algorithm for computing the CoM is wrong
you need to compute:

CoM = Sum_i(m_i*x_i) / Sum_i(m_i)

but you do compute:

Sum_i( Sum_i(m_i) * x_i)

axel.

> Thanks
> kirtana 

-- 
Dr. Axel Kohlmeyer
akohlmey_at_gmail.com  http://goo.gl/1wk0
Institute for Computational Molecular Science
Temple University, Philadelphia PA, USA.