Dear All
It is unfortunate that the questions that were asked by Mark appeared to
be phrased is such a way to get the responses that were obtained rather
than discussing the real issues behind the questions, or whether they
reflected the information in the Review. As such, I feel compelled
to reply to these comments as they are not an accurate reflection of what
was detailed in my article in Coordination Chemistry Review. While
scientific debate is welcome, as are corrections to errors, I believe
that is important that these information sites should not be used to give
incorrect information as to the contents of a paper.
1. Nowhere in this Coord Chem Rev article did I state that the SS
resolution equation does not apply to paths involving MS contributions,
so the question was incorrectly phrased, as were the responses that claim
the paper was incorrect in saying this. What I did say was that MS
analyses can distinguish metal-ligand bond lengths that differ by less
than the resolution imposed by this equation if the MS pathways from the
different ligands are sufficiently different (within the inherent Fourier
transform resolution) to get around this problem. There are
numerous examples in the literature where metal-ligand bond lengths that
differ by less than the SS resolution have been distinguished in this
manner and have been verified by comparing crystal structure information
with those obtained from MS analyses.
Here is a direct copy of the relevant section from the paper, so I am
uncertain as to how the contributors to the e-mails can come to the
conclusion that I stated that the equation does not apply to resolution
in MS paths. Clearly, I have stated that it is only when this
resolution is sufficiently different in other atoms of the restrained
ligands that metal-ligand bond lengths that have differences less than
the SS resolution of bond lengths can be distinguished (the article only
talked about ligands treated as restrained entities).
This limit on the resolution of distances arises because the larger
the k range, the greater the separation of the individual oscillations at
the end of the k range. While this equation is often quoted, the
resolution of the peaks in the FT corresponding to different shells
improves as the temperature is lowered due to reduction in the
Debye-Waller factors, so it should only be taken as a reasonable
guide. Thus the use of as large a k range as possible not only
improves the determinacy of the problem, but it also improves the
precision and accuracy to which individual metal-ligand bond lengths are
determined, and whether individual bond lengths can be resolved.
With the typical k ranges used in SS XAFS analysis, M-L bond differencs
have to differ by 0.1-0.2 Å for the oscillations in the XAFS to be
sufficiently resolved to distinguish between these two bond
lengths. By contrast, MS analysis of XAFS data has the ability
to distinguish between metal-ligand bond distances that differ by a
factor that is less than the ~0.1 Å limit imposed by the SS resolution,
provided that the groups to which the ligand donor atoms are attached
have quite different MS contributions, which is often the case. The
differentiation of M-L bond distances that are less than the resolution
obtainable with SS analysis relies on the MS contributions of other atoms
within the ligands, since these normally have sufficiently different
frequencies of oscillations in the XAFS so that they can be
resolved.
The MS contributions are most important in the low k range [18] and
hence, the most accurate and precise bond length determinations in terms
of resolution of different shells (and three-dimensional structural
determination) will be obtained when both a large k range and all of the
low k range data are used in the fitting procedure.
What do the above comments mean in practice? It means that if a
complex has ligands all of the same type and angles that involve the
M-ligand bond and the ligand are essentially the same, then MS analysis
cannot improve over SS on separating bond distances that differ by less
than the SS resolution in the FT. However, if ligands of the same
type are at different angles with respect to the metal-ligand axes or
have different ligands adjacent or opposite to them, or if different
ligands are at similar distances, then there is almost certainly a range
of MS paths that have half distances that differ by more than the
resolution given by the equation and, hence, the individual metal-ligand
distances can be distinguished by the use of MS fitting within the
constraints of the equation that was given, when they cannot be
distinguished by SS fitting.
2. With regard to point 2, yes the equation should have been more
accurately described as the number of independent observations rather
than the number of independent points in reference to equation 24 in the
review. The number of independent observations is a combination of
the number of independent points in the EXAFS data plus the number of
independent observations obtained from crystallography that are included
in the restrained model, as described in the Binsted, Strange and Hasnain
article. The equation (or variations of it, as pointed out by the
comments below) is, however, appropriate for use in estimating the value
of the number of independent observations that are included in restrained
MS modeling for fits to EXAFS data. This is not only used in such
EXAFS analyses but also extensively, and for a long time, in protein
crystallography to increase the degree to which a fit is overdetermined
or to prevent an underdetermined fit to the data. As pointed out by
Matt, this is often used for relatively rigid ligands, such as
imidazoles; it can also be used for flexible ligands where the internal
bond lengths vary over a narrow range that is defined by X-ray
crystallography, but one of more angle(s) have significant
flexibility. For the former case, the whole ligand is moved as an
essentially rigid entity, in much the same way as a single atom in the
fitting procedure. The essentially fixed (within small ranges) bond
angles and bond lengths involving this group lead to the extra term in
the equation that is the number of independent observations that are
included in the model and, in a restrained model, this increases the
value of N/p, even though the value of p does not change (i.e., the
positional parameters within the imidazole, for instance, are also fitted
but within a very narrow range that is consistent with the variation
observed in accurate X-ray crystallography structures). If these
bond lengths and angles (and/or Debye-Waller values) were constrained,
then the value of p would also decrease making the problem more
overdetermined. The Debye-Waller factors can be calculated for
atoms in a ring in order to reduce the value of p, but extreme care has
to be taken in the use of these values in constrained fits, since they
take no account of disorder problems in the crystal or solution and,
hence, could result in constrained values that are artificially low and
which would lead to problems in the fitting procedure.
The answer to the question is that the Stern paper discusses the number
of independent points in the EXAFS data and applies to both SS and MS
fitting. However, for restrained MS fits, the number of independent
observations that should be used in assessing the degree to which the
problem is determined in the fitting procedure is that described in the
Binsted, Strange and Hasnain paper (or something similar). The
latter type of fitting was that described in the Coordination Chemistry
Rev article and presumably has been used in the work described by
Mark.
All the best
Peter
HI Mark,
> Any thoughts and opinions would be greatly appreciated as this
relates directly to corrections suggested to be made to my PhD
thesis!
This is going to re-iterate most of what Bruce said, but since you
asked:
> 1)Does the equation for bond distance resolution (r = pi/2deltak)
only apply to SS?
No. It applies to all EXAFS.
> I have held the opinion that this can be applied to MS
analysis
You have been right.
> however I have recently been informed that this equation does not
correctly describe distance > resolution in MS analyses. The paper in
Coord. Chem. Rev. 2005, 249, 141-160 describes this > and is this
concordant with the views of the wider EXAFS community?
You were mis-informed. This paper is profoundly wrong, and is
not
concordant with the views of the wider EXAFS community, as defined
by
the standards and criteria documents at
http://www.i-x-s.org/OLD/subcommittee_reports/sc/err-rep.pdf
This paper states that the number of independent points in an XAFS
data set is:
N_i = [ 2*(rmax - rmin) * (kmax -kmin) / pi
] + Sum D*(N-2) + 1
Here rmin,rmax,kmin, and kmax are the spectral ranges. I cannot
tell
what the sum is over, but
D is the "dimension with a restrained part of the model (ie, three
for
a three-dimensional model)"
and N is "the number of independent atoms within the restrained
group
of the model".
The ( Sum D*(N-2) ) term asserts that the number of
independent
points in the data is dependent on the model. This is
complete
nonsense.
For what it's worth, the standards and criteria report cited above
recommends using
N_i = [ 2*(rmax - rmin) * (kmax -kmin) /
pi ]
rounded to the nearest integer. The report is has a bit more
to say,
but note that it is carefully (and deliberately) silent on
"+1", "+2",
etc. This is because N_i is an estimate of the maximum number
of
parameters that can be extracted from a signal. If you're
quibbling
whether to add 1 or 2 to this number, it probably means you
should
really subtract 4.
Now, one may apply a variety of modeling approaches (tricks??
assumptions??) to the analysis of multiple scattering in highly
constrained three-dimensional models that are often associated with
organo-metallics. For example, a histidine ring attached to a
metal
will give multiple scattering, and you can usually assert that the
ring is rigid, though you may need to refine its location and
orientation relative to the metal. That makes the bond
distances and
angles (and MS amplitudes) for all the scattering paths from this
ring
all dependent on a reduced number of variables. It does not
add
information to the data.
Cheers,
--Matt
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
X-Account-Key: account2
X-Mozilla-Keys:
Return-Path: <ifeffit-bounces@millenia.cars.aps.anl.gov>
X-Original-To: j.aitken@chem.usyd.edu.au
Delivered-To: aitken_j@chem.usyd.edu.au
Received: from localhost (unknown [127.0.0.1])
by
localhost.chem.usyd.edu.au (Postfix) with ESMTP id 3F99881B8;
Thu,
7 Jun 2007 07:04:39 +1000 (EST)
X-Virus-Scanned: amavisd-new at chem.usyd.edu.au
Received: from tismis.chem.usyd.edu.au ([127.0.0.1])
by
localhost (tismis.chem.usyd.edu.au [127.0.0.1]) (amavisd-new, port
10024)
with ESMTP
id FIIBlvbv0FPG; Thu, 7 Jun 2007 07:04:36 +1000 (EST)
Received: from millenia.cars.aps.anl.gov (millenia.cars.aps.anl.gov
[164.54.160.111])
by
tismis.chem.usyd.edu.au (Postfix) with ESMTP id 1AFAB8187;
Thu,
7 Jun 2007 07:04:35 +1000 (EST)
Received: from millenia.cars.aps.anl.gov (localhost.localdomain
[127.0.0.1])
by
millenia.cars.aps.anl.gov (8.13.8/8.13.8) with ESMTP id
l56L3Xc7017917;
Wed, 6 Jun
2007 16:03:34 -0500
Received: from polyhymnia.mr.aps.anl.gov (polyhymnia.mr.aps.anl.gov
[164.54.244.23])
by
millenia.cars.aps.anl.gov (8.13.8/8.13.8) with ESMTP id
l56L3WHx017914
for
<ifeffit@millenia.cars.aps.anl.gov>; Wed, 6 Jun 2007 16:03:32
-0500
Received: from localhost (localhost [127.0.0.1])
by
polyhymnia.mr.aps.anl.gov (8.13.8/8.13.8/Debian-3) with ESMTP id
l56L0im6015827;
Wed, 6 Jun 2007 16:00:44 -0500
From: Bruce Ravel <bravel@anl.gov>
Organization: Molecular Environmental Science Group at Argonne National
Lab
To: XAFS Analysis using Ifeffit
<ifeffit@millenia.cars.aps.anl.gov>
Date: Wed, 6 Jun 2007 16:00:43 -0500
User-Agent: KMail/1.9.6
References: <200706061213.DGT11246@auummr1om07.three.com.au>
In-Reply-To: <200706061213.DGT11246@auummr1om07.three.com.au>
MIME-Version: 1.0
Content-Disposition: inline
Message-Id: <200706061600.43646.bravel@anl.gov>
Cc: MarkBondin <mibondin@three.com.au>
Subject: Re: [Ifeffit] bond distance resolution and correlation of
parameters
in MS analyses?
X-BeenThere: ifeffit@millenia.cars.aps.anl.gov
X-Mailman-Version: 2.1.9
Precedence: list
Reply-To: Bruce Ravel <bravel@anl.gov>,
XAFS Analysis using Ifeffit
<ifeffit@millenia.cars.aps.anl.gov>
List-Id: XAFS Analysis using Ifeffit
<ifeffit.millenia.cars.aps.anl.gov>
List-Unsubscribe:
<http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit>,
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=unsubscribe>
List-Archive:
<http://millenia.cars.aps.anl.gov/pipermail/ifeffit>
List-Post:
<mailto:ifeffit@millenia.cars.aps.anl.gov>
List-Help:
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=help>
List-Subscribe:
<http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit>,
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=subscribe>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Sender: ifeffit-bounces@millenia.cars.aps.anl.gov
Errors-To: ifeffit-bounces@millenia.cars.aps.anl.gov
On Wednesday 06 June 2007, MarkBondin wrote:
> 2) What is the actual equation which defines the determinacy of a
fitting
> procedure and does this differ between SS and MS analyses? For MS
analyses
> there has been a recently published equation (Coord. Chem. Rev.
2005, 249,
> 141-160) which takes into acount the number of dimensions used in
the
> analyses and is this only relevant to MS analyses of data? This has
also
> been expressed in the paper by Binsted (Biochemistry. 1992,
31,
> 12117-12125) however a different equation has been detailed by Stern
(Phys.
> Rev. B. 1993, 48, 9825-9827) which I have been using as a guide in
my MS
> analyses. Is this acceptable?
1. I don't know what you mean by the word
"determinacy". In any
case, I thought I made it clear in my last post that,
in my
opinion, the differences between SS and MS analysis
are in the
physical interpretation and not in the statistical
interpretation.
Feff, Ifeffit, and Artemis certainly go to great
lengths to
downplay the differences between SS and MS paths in
the context of
the formalism of the theory and analysis, instead
emphasizing
their differences only in the context of physical
interpretation.
2. Argonne's library only has access to the last year of
Coord. Chem. Rev. and I don't have time this week to
go fetch it
from the stacks. So I cannot comment on that
paper.
3. The paper by Stern should be read with some care. The
argument Ed
makes in that paper can only be true in the case of a
perfectly
packed signal. EXAFS data, although treated as
signal processing
problem, is never perfectly packed. The Nyquist
criterion is an
upper bound on the information content, but the actual
content of
the data is always somewhat less. There are some
very fine papers
by Rossner and Krappe about using Baysian techniques
to find the
actual information content of the EXAFS signal.
The executive
summary is that if think you need Ed's magic
"+2", you are
probably overusing the information content of your
data.
Most of us here in this list aren't as careful in
practice as all
that Baysian stuff. In general, one tries to
stay "well below"
the Nyquist upper bound. If your fitting
parameters make sense
physically, if the correlations are not "too
high", and if the
error bars on your parameters are not "too
big", then you are
probably not overusing the information content of your
data.
What is "too high" and "too
big"? Well, I am purposefully using
squishy language. It is kind of difficult to use
Gaussian
statistical techniques on EXAFS data, despite the fact
that that's
exactly what Ifeffit does. The reason is that
Gaussin statistics
presumes that your measurememt errors are statistical
and normally
distributed. In practice, exafs analysis is
dominated by
systematic uncertainties. Things like detector
or sample
non-linearities and the approximations made by Feff
are much
bigger sources of error than shot noise for most
experiments.
Most of those systematic problems are present in your
analysis,
but I have no idea how you could possibly quantify
them. Hence I
find myself using squishy language to discuss fit
statistics.
Read the papers by the frequent contributors to this
mailing
list. Scott Calvin and Shelly Kelly in
particular are careful
EXAFS practitioners who work on tough analysis
problems and deal
well with these issues. Doing what they do may
not be as right as
possible, but it certainly ain't wrong.
HTH,
B
--
Bruce Ravel ----------------------------------------------
bravel@anl.gov
Molecular Environmental Science Group, Building 203, Room
E-165
MRCAT, Sector 10, Advanced Photon Source, Building 433, Room
B007
Argonne National
Laboratory phone and
voice mail: (1) 630 252 5033
Argonne IL 60439,
USA
fax: (1) 630 252 9793
My homepage:
http://cars9.uchicago.edu/~ravel
EXAFS software: http://cars9.uchicago.edu/~ravel/software/
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Return-Path: <ifeffit-bounces@millenia.cars.aps.anl.gov>
X-Original-To: j.aitken@chem.usyd.edu.au
Delivered-To: aitken_j@chem.usyd.edu.au
Received: from localhost (unknown [127.0.0.1])
by localhost.chem.usyd.edu.au (Postfix) with ESMTP id A3DA68195;
Wed, 6 Jun 2007 22:51:48 +1000 (EST)
X-Virus-Scanned: amavisd-new at chem.usyd.edu.au
Received: from tismis.chem.usyd.edu.au ([127.0.0.1])
by localhost (tismis.chem.usyd.edu.au [127.0.0.1]) (amavisd-new, port 10024)
with ESMTP id jldjzjMckVWz; Wed, 6 Jun 2007 22:51:47 +1000 (EST)
Received: from millenia.cars.aps.anl.gov (millenia.cars.aps.anl.gov [164.54.160.111])
by tismis.chem.usyd.edu.au (Postfix) with ESMTP id 75D278160;
Wed, 6 Jun 2007 22:51:47 +1000 (EST)
Received: from millenia.cars.aps.anl.gov (localhost.localdomain [127.0.0.1])
by millenia.cars.aps.anl.gov (8.13.8/8.13.8) with ESMTP id l56CnoPv011737;
Wed, 6 Jun 2007 07:49:51 -0500
Received: from polyhymnia.mr.aps.anl.gov (polyhymnia.mr.aps.anl.gov
[164.54.244.23])
by millenia.cars.aps.anl.gov (8.13.8/8.13.8) with ESMTP id
l56CnmLn011734
for <ifeffit@millenia.cars.aps.anl.gov>; Wed, 6 Jun 2007 07:49:48 -0500
Received: from localhost (localhost [127.0.0.1])
by polyhymnia.mr.aps.anl.gov (8.13.8/8.13.8/Debian-3) with ESMTP id
l56Cl1cT014080; Wed, 6 Jun 2007 07:47:01 -0500
From: Bruce Ravel <bravel@anl.gov>
Organization: Molecular Environmental Science Group at Argonne National Lab
To: ifeffit@millenia.cars.aps.anl.gov
Date: Wed, 6 Jun 2007 07:47:01 -0500
User-Agent: KMail/1.9.6
References: <200706061213.DGT11246@auummr1om07.three.com.au>
In-Reply-To: <200706061213.DGT11246@auummr1om07.three.com.au>
MIME-Version: 1.0
Content-Disposition: inline
Message-Id: <200706060747.01272.bravel@anl.gov>
Cc: MarkBondin <mibondin@three.com.au>
Subject: Re: [Ifeffit] bond distance resolution and correlation of parameters
in MS analyses?
X-BeenThere: ifeffit@millenia.cars.aps.anl.gov
X-Mailman-Version: 2.1.9
Precedence: list
Reply-To: Bruce Ravel <bravel@anl.gov>,
XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
List-Id: XAFS Analysis using Ifeffit <ifeffit.millenia.cars.aps.anl.gov>
List-Unsubscribe: <http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit>,
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=unsubscribe>
List-Archive: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit>
List-Post: <mailto:ifeffit@millenia.cars.aps.anl.gov>
List-Help: <mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=help>
List-Subscribe: <http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit>,
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=subscribe>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 8bit
Sender: ifeffit-bounces@millenia.cars.aps.anl.gov
Errors-To: ifeffit-bounces@millenia.cars.aps.anl.gov
On Wednesday 06 June 2007, MarkBondin wrote:
1)Does the equation for bond distance resolution (r = pi/2deltak) only
apply to SS? I have held the opinion that this can be applied to MS
analysis however I have recently been informed that this equation does not
correctly describe distance resolution in MS analyses. The paper in Coord.
Chem. Rev. 2005, 249, 141-160 describes this and is this concordant with
the views of the wider EXAFS community?
Mark,
Strictly speaking, that equation doesn't have anything to do with
EXAFS. That is the equation that tells you what your Fourier
component resolution is in a general Fourier analysis problem. It
just so happens that, in the case of single scattering EXAFS analysis,
that equation is easily interpreted in terms of photoelectron
wavenumber k and SS path length r.
The equation is neither different nor incorrect for MS analysis.
That's true becuase MS analysis isn't any different from SS analysis.
In either case, you do a Fourier transform. In either case, you
attempt to model Fourier components using the contributions from some
number of scattering geometries as computed by theory. In either
case, you are asking yourself if you can actually resolve small
differences in phase of the various things that contribute to the fit.
The only difference lies in how you *interpret* the physical meaning
of the Fourier components. And even then, things aren't so very
different. In the case of SS analysis, you assert that the R axis is
a measure of "bond length" while for MS analysis the R axis is a
measure of "half path length" -- acknowledging, of course, that there
is a phase shift in the EXAFS equation such that the R axis actually
measures something a bit shorter than the bond or half path length.
Off the top of my head, I don't recall the paper you cite and I most
certainly cannot speak for the "wider EXAFS community". Speaking for
myself, the physical interpretation of the equation for Fourier
component resolution may change when you consider MS paths, but to
claim that a property of the Fourier transform somehow becomes invalid
when you change the details of the fitting model is just silly.
B
--
Bruce Ravel ---------------------------------------------- bravel@anl.gov
Molecular Environmental Science Group, Building 203, Room E-165
MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007
Argonne National Laboratory phone and voice mail: (1) 630 252 5033
Argonne IL 60439, USA fax: (1) 630 252 9793
My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Return-Path: <ifeffit-bounces@millenia.cars.aps.anl.gov>
X-Original-To: j.aitken@chem.usyd.edu.au
Delivered-To: aitken_j@chem.usyd.edu.au
Received: from localhost (unknown [127.0.0.1])
by localhost.chem.usyd.edu.au (Postfix) with ESMTP id B52C68192;
Wed, 6 Jun 2007 22:34:01 +1000 (EST)
X-Virus-Scanned: amavisd-new at chem.usyd.edu.au
Received: from tismis.chem.usyd.edu.au ([127.0.0.1])
by localhost (tismis.chem.usyd.edu.au [127.0.0.1]) (amavisd-new, port 10024)
with ESMTP id Bt1hFrBPzdYS; Wed, 6 Jun 2007 22:34:01 +1000 (EST)
Received: from millenia.cars.aps.anl.gov (millenia.cars.aps.anl.gov [164.54.160.111])
by tismis.chem.usyd.edu.au (Postfix) with ESMTP id 8255C8160;
Wed, 6 Jun 2007 22:34:00 +1000 (EST)
Received: from millenia.cars.aps.anl.gov (localhost.localdomain [127.0.0.1])
by millenia.cars.aps.anl.gov (8.13.8/8.13.8) with ESMTP id l56CVHTh011592;
Wed, 6 Jun 2007 07:31:35 -0500
Received: from petasus.aps.anl.gov (petasus.aps.anl.gov [164.54.146.72])
by millenia.cars.aps.anl.gov (8.13.8/8.13.8) with ESMTP id
l56CDQCl011430
for <ifeffit@millenia.cars.aps.anl.gov>; Wed, 6 Jun 2007 07:13:26 -0500
Received: from herald.aps.anl.gov (herald.aps.anl.gov [164.54.50.61])
by petasus.aps.anl.gov (8.13.7/8.13.7) with ESMTP id l56CDPH7020467
for <ifeffit@millenia.cars.aps.anl.gov>;
Wed, 6 Jun 2007 07:13:25 -0500 (CDT)
Received: from iota.aps.anl.gov (iota.aps.anl.gov [164.54.56.65])
by herald.aps.anl.gov (8.13.7/8.13.7) with ESMTP id l56CDDTr015527
for <ifeffit@millenia.cars.aps.anl.gov>;
Wed, 6 Jun 2007 07:13:13 -0500 (CDT)
Received: from auummr1om07.three.com.au (smtpout3.three.com.au [202.124.68.59])
by iota.aps.anl.gov with ESMTP id l56CDBRl028957
for <ifeffit@millenia.cars.aps.anl.gov>;
Wed, 6 Jun 2007 07:13:12 -0500 (CDT)
Received: from auumgu1pr04.three.com.au (auummr1vp02.three.com.au
[10.176.58.185]) by auummr1om07.three.com.au (MOS 3.5.5-GR)
with ESMTP id DGT11246; Wed, 6 Jun 2007 22:13:03 +1000 (EST)
Message-Id: <200706061213.DGT11246@auummr1om07.three.com.au>
Content-Disposition: inline
MIME-Version: 1.0
X-Mailer: Perl5 Mail::Internet v1.40
From: MarkBondin <mibondin@three.com.au>
To: "ifeffit@millenia.cars.aps.anl.gov" <ifeffit@millenia.cars.aps.anl.gov>
Date: Wed, 06 Jun 2007 22:13:03 +1000 (EST)
X-Priority: 3 (Normal)
X-MIME-Autoconverted: from quoted-printable to 8bit by
millenia.cars.aps.anl.gov id l56CDQCl011430
X-Mailman-Approved-At: Wed, 06 Jun 2007 07:31:16 -0500
Subject: [Ifeffit] bond distance resolution and correlation of parameters
in MS analyses?
X-BeenThere: ifeffit@millenia.cars.aps.anl.gov
X-Mailman-Version: 2.1.9
Precedence: list
Reply-To: XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
List-Id: XAFS Analysis using Ifeffit <ifeffit.millenia.cars.aps.anl.gov>
List-Unsubscribe: <http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit>,
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=unsubscribe>
List-Archive: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit>
List-Post: <mailto:ifeffit@millenia.cars.aps.anl.gov>
List-Help: <mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=help>
List-Subscribe: <http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit>,
<mailto:ifeffit-request@millenia.cars.aps.anl.gov?subject=subscribe>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 8bit
Sender: ifeffit-bounces@millenia.cars.aps.anl.gov
Errors-To: ifeffit-bounces@millenia.cars.aps.anl.gov
Hi, I would like general opinions of some EXAFS practioners in regards to the most widely accepted methods for bond distance resolution and correlation of parameters in both single scattering (SS) and multiple scattering (MS) EXAFS refinements and if these can be said to differ?
1)Does the equation for bond distance resolution (r = pi/2deltak) only apply to SS? I have held the opinion that this can be applied to MS analysis however I have recently been informed that this equation does not correctly describe distance resolution in MS analyses. The paper in Coord. Chem. Rev. 2005, 249, 141-160 describes this and is this concordant with the views of the wider EXAFS community?
2) What is the actual equation which defines the determinacy of a fitting procedure and does this differ between SS and MS analyses? For MS analyses there has been a recently published equation (Coord. Chem. Rev. 2005, 249, 141-160) which takes into acount the number of dimensions used in the analyses and is this only relevant to MS analyses of data? This has also been expressed in the paper by Binsted (Biochemistry. 1992, 31, 12117-12125) however a different equation has been detailed by Stern (Phys. Rev. B. 1993, 48, 9825-9827) which I have been using as a guide in my MS analyses. Is this acceptable?
Any thoughts and opinions would be greatly appreciated as this relates directly to corrections suggested to be made to my PhD thesis!
Many thanks,
Mark
_
Professor Peter A. Lay FAA
ARC Australian Professorial Fellow and Personal Chair in Inorganic Chemistry
Centre for Heavy Metals Research
School of Chemistry
The University of Sydney
NSW 2006
Australia
Tel: +61-2-9351 4269
Fax: +61-2-9351 3329