Subject: summary: charge in a point outside a molecule

From: Sofia Godinho <[email-masked]>
Date: Thu, 5 Apr 2001 15:19:16 +0100 (WEST)

I thank all the people who have tried to help me.
Summary of the answers order by date of arrival obtained to the following
message:

* Dear CCLers,
* I wonder if anyone can teach me how to calculate, with Gaussian, the
* charge in a specific point of the space outside a molecule. Let's say
* in a point defined by x,y,z coordinates away from that molecule. 
* Thank you in advance for your help,
*        Sofia Godinho

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From: Alexander Wittkopp <[email-masked]>
Hi,

   I don't think you can place a charge somewhere in space, but since
elektrones can only be where basis sets are you just have to define an
origin for those and set the over all charge to -1, or whatever you like.
E.g. you could place a helium atom where you want to place your charge
and set the Nuc.-charge of He to 0 (keyword: massage). With Gen you can
modify the shape of this charge, by modifying the basis set of He.

This procedure does not guarantee that your overall charge lands on this
'ghost basis set', it will be distributed all along the whole molecule but
it is the only method I can think of.  This method has been used to
determine the 'stability' of solvated electrons. (I think T. Clark, end of
the 80s)

Cheers,
        Alexander

PS: Let me know if somebody has nicer ideas!
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From: "Shobe, Dave" <[email-masked]>

Your question is open to interpretation:

If you mean to calculate the "charge" at a point in space, which is caused
by the electron density and electric field of the molecule, the answer is
zero (or "infinitesimal" if you like).  However, there is such a thing as
the charge *density* at a point, which is (in absolute values anyway) the
same as the electron density, and which *integrated over a region* of
space
gives a charge for that region. (Of course I am talking about points away
> from the nuclei here).  There is also an electrostatic potential and an
electric field at that point.  The relevant Gaussian keywords are CUBE and
PROP.

If you mean to do a calculation in which "external" point charges have
been
added, you can use the CHARGE keyword.  

--David Shobe
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From: Andrea G. Alparone <[email-masked]>
Dear Sofia,

You could use the Gaussian "cube" keyword to calculate electron densities
or electrostatic potentials.

An example for calculating the electron density can be the following
(the density value at x = a, y = b and z = c cartesian-coordinates will
be
reported in the name.cube file):

# hf / sto-3g cube=(cards,density)

No title specified

 0   1
    a                              0.000000    1.227945    0.000000
    b                              2.063432   -0.613973    0.000000
    c                             -2.063432   -0.613973    0.000000

name.cube
  -51        a            b           c
   1          0.0         0.0         0.0
   1          0.0         0.0         0.0
   1          0.0         0.0         0.0

Of course you can also evaluate grids of properties.

For details I advice you to see the Gaussian user's reference or see at
the following URL http://www.gaussian.com/00000430.htm.

Best regards

Andrea.
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From: Laurence Cuffe <[email-masked]>
This is by definition Zero. However if you are looking for the electron 
density in a given volume near x,y,z try using the Cube command 
which can give you the electron density at a grid of points around 
the molecule.  I hope this helps  
Laurence Cuffe
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From: Artem Masunov <[email-masked]>
Sofia,
Just put CHARGE keyword in command string, and
x y z q
after the blank line at the end of the input
This will work for SP calculations only.
You can also use a gaussian charge distribution
instead of the point charge.
See the manual
http://www.gaussian.com/00000424.htm
not much details there, unfotunately

Hope this hepls,
Artem
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