Subject: CCL:helix dipole

From: <[email-masked]>
Date: Mon, 25 Apr 1994 18:29:37 -0400 (EDT)

>On Fri, 22 Apr 1994, Srinivasan S. Iyengar wrote:
>
>> 
>>In his mail Rafael Najmanovich says :
>>>
>>>         For each pair of opposite charges in the helix you have a dipole
>>> momentum vector that points from the positive to the negative charge so I
>>> suppose the total dipole momenta vector of that helix will be the
>>> vectorial sum of all these vectors. How to calculate it in practice (that
>>> is, if there are approximation methods or computer algorhitms) I don't know.
>>>
>> 
>>    I beg to differ. I would rather -
>>  (a) Find the center of mass of all the positive charges.
>>  (b) Find the center of mass of all the negative charges.
>>  (c) Find the dipole moment between these two points.
>> 
>> Srinivasan Iyengar
>> 
>	I'm not sure but I think you'll problably get the same resultant
>vector.
>
>	Rafael najmanovich

As was noted by a previous poster, the definition of the dipole moment vector
is:

      mu_x = SUM_i q_i * (x_i - x0)
      mu_y = SUM_i q_i * (y_i - y0)
      mu_z = SUM_i q_i * (z_i - z0)

where mu_x, mu_y, mu_z are the components of the dipole moment vector mu along
the x, y, z axes (respectively); q_i is the charge of entity i; x_i, y_i, z_i
are the cartesian coordinates of entity i; and x0, y0, z0 are the cartesian
coordinates of a reference point.  For electrically neutral molecules mu is
independent of the reference point.  Otherwise the molecule's center of mass
is conventionally used as (x0,y0,z0).  The molecule's principle axes provide
three convenient orthogonal vectors that may be used to define the x, y, and
z axes.

The equations above are the correct ones to use, and are just as easy (if not
easier) to program than some of the schemes provided by other posters (I assume
the original poster did not want to do these calculations by hand).

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Gregory King                             Internet: [email-masked]
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