Model molecular systems with possible proton transfer

It is also possible to treat solvent explicitly, that is, to include one or several solvent molecules in the QM description of the system. As most solvents are fairly small molecules, this might seem like an affordable, desirable, and more accurate approach that could be combined with continuum models in a scheme where the first shell of solvent is described explicitly and the second shell is treated with a continuum. Hybrid techniques in which one part of a system is treated with one level of theory and another part (e.g., the solvent) with another, lower, level of theory such as molecular mechanics (in which case one has a QM/MM method) also seem well suited to describe solutions. Indeed, this type of approach can be very useful in many cases. Treating at least some solvent explicitly is of course necessary when the solvent participates directly in a reaction, by proton transfer or by bonding to a metal, for example. 

Malonaldehyde with its intra-molecular proton transfer is another important model system to study these kind of processes and is hence a well-studied molecule. Experimental IR and Raman spectra have been recorded over the last decades and allow a detailed evaluation of full-dimensional (21-D) model calculations which only became possible in the recent years. In the present contribution we have... 

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