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

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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) 1 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 1 quite different conclusions [36,41].” Firstly the results opencircles. TheinsetshowsthehighQregioninmoredetail. v reportedinreferences36[2]and41[3]doindeedoriginate Curvesare shifted vertically for clarity. 7 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 1 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]. 2 [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).

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