From:	K Bryson <kb7 "at@at" tower.york.ac.uk>
Date:	Tue, 3 Aug 1993 11:22:25 +0100 (BST)
Subject:	Re: ESP-fitted Point Charges

	Hi !,

On Tue, 3 Aug 1993 apa()at()pop.cc.adfa.oz.au wrote:

>                         .........
> I would like some clarification on this highlighted point. The ESP'ified MOPAC
> that I'm using will produce a set of atom-centered point charges which are
> least-squares-fitted to reproduce the MOPAC semiempirical electrostatic
> potential.  If the DIPOLE or DIPX/DIPY/DIPZ keywords are used, the fitting
> procedure will also be constrained to reproduce the dipole moment (or x-,
> y- or z- components).
> MOPAC seems only to allow this constraint if the molecule is neutral.
>
> Why is this?
>
> Taking the ammonium ion as an example, why cannot the semiempirical esp be
> fitted to a set of 5 N- and H-centered point-charges, chosen so that the
> overall charge is +1 and dipole moment is zero in x-, y- and z- directions?

	Hi Alan,
		The reason that Mopac doesn't allow fitting dipole moments
	to charged molecules is because they are not well-defined.
		The dipole moment of an ammonium ion is zero if you
	take the nitrogen as an origin due to symmetry, but if you calculate
	it's dipole moment taking an origin which is 1 Angstrom away from
	the nitrogen then you will get a dipole moment of 1 Angstrom*e,
	in general any charged molecule will have a 'center of electric
	charge' ( analogous to a center of mass ) where it's dipole moment
	is zero and for positions at a distance d from this center of charge
	it's dipole will be dQ where Q is the charge on the molecule.
		For neutral molecules this is OK since the dipole moment
	turns out to be independent of the origin position, but for charged
	molecules this leads to ill-defined dipole moments. Hence the
	experimental dipole moment of a charged molecule is non-existent and
	so we have no experimental data to compare our ESP charges with,
	making the calculation of ESP charges for charged systems even more
	problematic ...


	Summary of Personal Mail for my posting.
	----------------------------------------

		I received two comments regarding my posting, one about
	a way around the static variable storage of MOPAC for machines
	without the static flag, and the other agreeing with my primary
	aminium charges using more sophisticated fitting techniques ...

-------------------------------------------------------------------------
From  :

    ...........    I was then made aware of a correction that possibly
needs to be made to the ESP code.  There should be a loop at or about
line 1117 in the module esp.f:-

C  SOLVE FOR THE CHARGES
     
C  SET THE CHARGES TO ZERO
      DO 65 I = 1,NATOM+4
      Q(I) = 0.0D0
65    CONTINUE

If this loop is not there I would advise adding it.  When I did this
it made the results more sensible.

I am not sure whether this code needs to be added to all implementations
of mopac, I think that it may be something to do with whether you use
the -static option when compiling on SGI machines. ..........

Peter Bladon

--------------------------------------------------------------------------
From dan { *at * } omega.chem.yale.edu :

   Hi!
> 	Primary aminium   NH_3
>            N = -0.40    H = +0.36
>       Secondary aminium   NH_2
>            N = +0.01    H = +0.27
>       ........

    Your primary aminium bears similarity to our OPLS potential
  function (N = -0.4, H=0.23) which was not derived using ESP
  formalisms, but rather by fitting ab initio water interaction
  energies and experimental free energies of hydration.  Your secondary
  aminium looks different, though, although I know less about our
  secondary and teriary potential functions.

    I only have experience with the CHELPG charges from G92 so I can't
  comment further as to their ability to work for charged systems. We
  have only recently started using them to supplement our charge fitting
  procedures.

    Dan Severance

----------------------------------------------------------------------------

 ----------~~~~~~------~~~~~~~~~~---------------~~~~~----------~~~~~--
    K.Bryson                 email: kb7 at.at tower.york.ac.uk
    Biophysics Group         Tel  : +44 904 430000 Extn. 2236
    Physics Department       Fax  : +44 904 432214
    University of York
    Heslington              "Molecular modelling of DNA and its
    YORK, UK                    interaction with small molecules."
    YO1 5DD
 -------------------~~~~~-------------~~~~--------~~~~--------~~~~~---




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