There are various routes for deriving partial radial distribution functions of disordered systems from experimental diffraction (and/or EXAFS) data. Due to limitations and errors of experimental data, as well as to imperfections of the evaluation procedures, it is of primary importance to confirm that the end result (partial radial distribution functions) and the primary information (diffraction data) are consistent with each other. We introduce a simple approach, based on Reverse Monte Carlo modelling, that is capable of assessing this dilemma. As a demonstration, we use the most frequently cited set of "experimental" partial radial distribution functions on liquid water and investigate whether the 3 partials (O-O, O-H and H-H) are consistent with the total structure factor of pure liquid D 2 O from neutron diffraction and that of H 2 O from X-ray diffraction. We find that while neutron diffraction on heavy water is in full agreement with all the 3 partials, the addition of X-ray diffraction data clearly shows problems with the O-O partial radial distribution function. We suggest that the approach introduced here may also be used to establish whether partial radial distribution functions obtained from statistical theories of the liquid state are consistent with the measured structure factors.

Following a fairly comprehensive study on popular interaction potentials of water (Pusztai et al., J. Chem. Phys., 2008, 129, 184103), here two more recent polarizable potential sets, SWM4-DP (Lamoureux et al., Chem. Phys. Lett., 2006, 418, 245) and BK3 (Kiss et al., J. Chem. Phys., 2013, 138, 204507) are compared to the TIP4P-2005 water potential (Abascal et al., J. Chem. Phys., 2005, 123, 234505) that had previously appeared to be most favoravble. The basis of comparison was the compatibility with the results of neutron and X-ray diffraction experiments on pure water, using the scheme applied by . The scheme combines the experimental total scattering structure factors (TSSF) and partial radial distribution functions (PRDF) from molecular dynamics simulations in a single structural model. Goodness-of-fit values to the O-O, O-H and H-H simulated PRDF-s and to the experimental neutron and X-ray TSSF provided a measure that can characterize the level of consistency between interaction potentials and diffraction experiments. Among the sets of partial RDF-s investigated here, the ones corresponding to the SWM4-DP potential parameters have proven to be the most consistent with the particular diffraction results taken for the present study, by a hardly significant margin ahead of BK3. Perhaps more importantly, it is shown that the three sets of potential parameters produce nearly equivalent PRDF-s that may all be made consistent with diffraction data at a very high level. The largest differences can be detected in terms of the O-O partial radial distribution function.

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Neutron- and x-ray weighted total structure factors of liquid water have been calculated on the basis of the intermolecular parts of partial radial distribution functions resulting from various computer simulations. The approach includes reverse Monte Carlo (RMC) modelling of these partials, using realistic flexible molecules, and the calculation of experimental diffraction data, including the intramolecular contributions, from the RMC particle configurations. The procedure has been applied to ten sets of intermolecular partial radial distribution functions obtained from various computer simulations, including one set from an ab initio molecular dynamics, of water. It is found that modern polarizable water potentials, such as SWM4-DP and BK3 are the most successful in reproducing measured diffraction data.

matematika-kémia szakos középiskolai tanár (Érdi Gárdonyi Géza Általános Iskola és Gimnázium) Témavezető: Jóvári Pál Ph.D., tudományos főmunkatárs (MTA Wigner Fizikai Kutatóközpont) ELTE TTK Kémia Doktori Iskola (iskolavezető: Prof. Inzelt György) Elméleti és fizikai kémia, anyagszerkezetkutatás program (programvezető: Prof. Surján Péter) 2016.