Protonated hydrates Monte Carlo calculations

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

Theoretical studies on protonated hydrates (PH) are illustrative of the progress realized in theoretical chemistry over several decades. The evolution of such studies is presented. The main methods used (quantum chemistry, Monte Carlo or Molecular Dynamics calculations...) and the problems encountered are briefly recalled. Some of the results obtained are commented. 

This short and very limited review gives an idea of the wide variety and complexity of the problems encountered in the study of protonated hydrates. This selection of topics is illustrative of the new possibilities offered by the present developments of theoretical treatments and shows the progress realised in this field, especially within the last few years. Besides the improvement of the accuracy of some of the methods used, in particular to treat the correlation contribution to the intermolecular energies, other categories of methods of lower accuracy bring other kinds of information. This has been exemplified by the results obtained from Monte Carlo and Molecular Dynamics calculations based on the use of DFT or analytical potentials. Though great care must be taken to the possibility of artefacts due to the approximations included in these treatments, such work is complementary to more accurate determinations. We can expect all these methods to be more and more coupled in the near future. 

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