Comment on "Oxygen as a Site Specific Probe of the Structure of Water and Oxide Materials", PRL 107, 144501 (2011)

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by  A. K. Soper

Comment on “Oxygen as a Site Specific Probe of the Structure of Water and Oxide
Materials”, PRL 107, 144501 (2011)
A.K.Soper
STFC Rutherford Appleton Laboratory, ISIS Facility,
Harwell Oxford, Didcot, Oxfordshire, OX11 0QX, UK∗
C.J.Benmore
Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Il 60439
2 (Dated: January 11, 2012)
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0
2
n A recent paper by Zeidler et al. [1] describes a neu- 0.04 0.01
tron scattering experiment on water in which oxygen
a isotope substitution is successfully achieved for the first FD(Q)
J 0.03
10 tfeimreen.t Doxiffyegreennciseostbopetewsegeinvescaatctoerminbginpaatitotnernosf wthitehOd-iOf- F(Q) 0 FH(Q)
andO-H(orO-D)structurefactors,andthemethodele- 0.02
ph] gfeacnttslyasmsoinciiamteisdeswsiothmeneoufttrhoenpsrcoabttleemrinagticfrionmelahsytidcritoygeenf-. F(Q) 0.01 10 Q [ 1Å5−1] 20 FD(Q)
- Particular conclusions of the new work are that the OH
m bond length in the H2O molecule is about0.005˚Alonger 0 FH(Q)
e thanthesamebondinD2O,andthatthehydrogenbond
h
peaks in both liquids are at about the same position.
c −0.01
s. Notwithstanding the substantial progress demon- 0 5 10 Q [Å−1] 15 20 25
c strated by the new work, the comparison with our own
i
s results [2] by Zeidler et al. is in our opinion misleading.
y The lastparagraphof[1]statesthatthe new experimen- FIG. 1: Simulated first order difference functions (solid
h tal results “...originate directly from measured data sets lines), ∆FD(Q) = 0.0059[SOD(Q)−1]+0.00262[SOO(Q)−
p 1](top),∆FH(Q)=−0.0033[SOH(Q)−1]+0.00263[SOO(Q)−
and not from a refinement of models using the diffrac-
[ 1](bottom), based on the EPSR simulations reported in [2].
tion data, wherein different starting points can lead to The oxygen isotope substitution data from [1] are shown as
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quite different conclusions [36,41].” Firstly the results opencircles. TheinsetshowsthehighQregioninmoredetail.
v
reportedinreferences36[2]and41[3]doindeedoriginate Curvesare shifted vertically for clarity.
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7 directly from measured datasets, as is abundantly clear
9 from the papers themselves. Equally the reported OH
1 bond lengths in [2] are a direct consequence of the par- This simulation is then used to claim that the hydrogen
.
1 ticulardiffractiondatapresented,ashighlightedinFig. 2 bond length in H2O is about the same length as that in
0 ofthatpaper, anddo notdepend onthe computer simu- D2O, just as in [2] the EPSRsimulation is used to claim
2 lation method used. Both papers use different empirical these bonds are different. Note however that the results
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potential structure refinement (EPSR) computer simu- [2] reproduce the change in x-ray scattering pattern be-
:
v lations to fit radiation scattering data and derive OO, tween heavy and light water [6], something that TTM
Xi OHandHHradialdistributionfunctionscompatiblewith potentials tend to underestimate [7].
thosedata. ComparingFig3. of[2]withFig. 6of[3]and Toillustratetheambiguitiesassociatedwithusingany
r
a Fig. 6 of [4], which include independent datasets from particular dataset to characterise the structure of wa-
bothx-rayandneutron(reactorandpulsed)sources,and ter we have constructed the oxygenfirst order difference
from “different starting points”, the radial distribution functions, ∆FH(Q) and ∆FD(Q) (Figure 1(a) of [1]) us-
functions for water extracted by this method from all ing the simulated OO and OH partial structure factors
thesedifferentdatasetsareinfactverysimilar. Butthey thatwereextractedinthepreviousanalysis[2],weighted
areofcoursenotidenticalandthisisknowntobedue to according to the new values of the oxygen isotope scat-
systematic uncertainties which are difficult to quantify, tering lengths, and without refinement against the new
particularlywherescatteringfromhydrogenorsmalliso- data (Figure 1). Although there is some disagreement
tope differences is involved [5]. Zeidler et al. distance in the region of the main peak near Q = 2˚A−1 in the
themselves from the EPSR approach, yet by choosing ∆FD(Q) function, it is striking how close the earlier
one potential, TTM3-F, out of several possibilities they EPSRsimulationsapproachthenewdata. Asimilardis-
have in fact performed what EPSR does automatically, crepancyin∆FD(Q)betweensimulationandexperiment
namely selected a potential that reproduces their data. near Q=2˚A−1 is observed in Figure 1(a) of [1].
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[4] A.K.Soper,J.Phys.Condens.Matter19,335206(2007).
[5] H.E.Fischer,A.C.Barnes,andP.S.Salmon,Rep.Progr.
Phys. 69, 233 (2006).
∗
[email protected] [6] R. T. Hart, C. J. Benmore, J. Neuefeind, S. Kohara,
[1] A. Zeidler, P. S. Salmon, H. E. Fischer, J. C. Neuefeind, B. Tomberli, and P. A. Egelstaff, Phys. Rev. Lett. 94,
J. M. Simonson, H. Lemmel, H. Rauch, and T. E. Mark- 047801 (2005).
land, Phys. Rev.Lett. 107, 145501 (2011). [7] F. Paesani and G. A. Voth, J. Phys. Chem. B 113, 5702
[2] A. K. Soper and C. J. Benmore, Phys. Rev. Lett. 101, (2009).
065502 (2008).
[3] A. K.Soper, Chem. Phys.258, 121 (2000).