Subject: (Summary): Some questions about MK, CHELP and CHELPG in G94

From: Pablo Vitoria Garcia <[email-masked]>
Date: Wed, 19 Feb 1997 10:48:05 +0000 (WET)

Hi,

Following this, the answers I got to the questions stated below.
Thank you very much to all who responded.
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(1) I understand that RMS and RRMS measure the agreement of the fitting 
procedure. But, are they internal to each method (MK, CHELP or CHELPG) or 
can they be compared? I mean, if I calculate ESP fitted charges with the 
three methods mentioned, can I assume that the best fit is the one with 
the lowest RMS?

(2) Does it make any sense to constrain the ESP calculated charges to fit the 
dipole moment for CHARGED molecules?
I am not sure about it, since the dipole moment value depends on the 
choice of origin for non-neutral molecules.

Thanks a lot for your help

Pablo

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From [email-masked] Feb 19 10:39:11 1997
Date: Fri, 14 Feb 1997 18:52:47 +0100
From: Konrad Hinsen <[email-masked]>

That is no problem, as long as you use the same origin consistently.
On the other hand, you normally don't have to constrain the dipole,
because it gets reproduced very well from the ESP fit. Even the
total charge normally gets reproduced rather well - the reason for
constraining it that it is often important to reproduce it exactly,
to ensure overall charge neutrality of a system.

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From [email-masked] Feb 19 10:39:22 1997
Date: Sun, 16 Feb 1997 18:59:43 +1100 (EST)
From: Jeremy R Greenwood <[email-masked]>

No answer to your question, but while we're on the subject:

I'm keen to find ways of interpreting dipole moment of a series of 
anions, in conjunction with looking at the way a series of carboxylate
bioisosteres interact with a charged or strongly polar receptor of
unknown structure. I know the *value* depends on the choice of origin, 
but what about the *direction* of the dipole vector? Can this be used 
as an indicator of the way in which an anion will prefer to align in 
a local field, or does it too depend on coordinate choice?

Also, are there well-established experimental techniques for measuring 
dipole direction, as opposed to value?

Thanks,

Jeremy

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From [email-masked] Feb 19 10:39:31 1997
Date: Mon, 17 Feb 1997 12:24:05 -0500
From: "Donald E. Williams" <[email-masked]>

Dear Pablo:
	It would not be correct to blindly assume that PD charges
with the lowest rms are best.  The main reason is the differing
choices of grids for the MEP.  Generally the rms (and rrms) is higher
if grid points are chosen closer to the molecule.
-Donald Williams

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Date: Sat, 15 Feb 97 10:40:10 NFT
From: [email-masked]
Subject: Fitted charges

Hola! Pablo:

To answer your to questions regarding ESP charges, first the `RMSD' re-
flects  the ability of the fitted charges  to reproduce the surrounding
electrostatic potentials. It is nothing else than the functional:

                                     2  1/2
{ 1/N Sum [ V(SCF) - Sum (q_i/r_pi) ]  }
       p              i

If your potential can be easily fitted by point charges,  than the RMSD
will be rather small. If, however,  there is a non-negligible contribu-
tion of atomic moments of higher orders, than the RMSD will increase.

Second,  constraining the fit to reproduce the dipole of a charged spe-
cies does not make sense.  For such species,  the dipole moment depends
on the orientation of the system. Besides and by and large, I never saw
any advantage to add such constraints. It unnecessarily biases your fit
which, if your potential is evaluated correctly, should be okay without
any other constraint than the simple q_total - Sum q_i = 0.
                                                i

You can look at a few refs. from us, which cover the above:

- Chipot, C.,  Maigret, B.,  Rivail, J.L. and Scheraga, H.A.,  J. Phys. 
  Chem., 1992, 96, 10276

- Angyan, J.G. and Chipot, C., Int. J. Quantum Chem., 1994, 52, 17

- Chipot, C. and Angyan, J.G., GRID Version 3.0: Point Multipoles Deri-
  ved From Molecular Electrostatic Properties,  1994, 14, QCPE No. 655,
  (Version 3.1 currently available)

I hope this helps... Best regards,

Chris
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Pablo Vitoria Garcia 
Departamento de Quimica Inorganica, Facultad de Ciencias
Universidad del Pais Vasco (UPV/EHU)
Apartado 644, E-48080 Bilbao
SPAIN
e-mail: [email-masked]
Phone: +34 4 4647700 Ext. 2450
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