An international team of scientists from Spain (Cristina Díaz of the UAM), The Netherlands (Geert-Jan Kroes), Norway (Roar Olsen), Argentina (Fabio Busnengo), and the United States (Daniel Auerbach) has shown in a paper recently published in Science, how the chemistry of surface reactions underpinning catalysis can be modelled accurately with computers.

Determining the mechanisms behind such surface reactions is more than a frivolous academic exercise; reactions of gas phase molecules with metal surfaces are of tremendous practical importance, as the production of most synthetic compounds involves such reactions. In fact, one of the achievements recognised by the 2007 Nobel Prize in Chemistry, awarded to Gerhard Ertl, was the detailed description of the sequence of elementary molecule-surface reactions by which vast quantities of ammonia are produced for fertiliser.

So far scientists were only able to predict the rate of molecule-surface reactions with semi-quantitative accuracy. But in a recent development, an important step has been taken towards a quantitative understanding of how molecules interact with surfaces. The team mentioned above, which includes Cristina Diaz of the Chemistry Department of the UAM, has shown, and published in Science, that an important class of molecule-surface reactions (dissociation of molecular hydrogen on metal surfaces) can now be modelled by computers “with chemical accuracy”. By chemical accuracy is meant that the interaction energy between the molecule and the surface has an error in it that is no greater than 1 kcal/mol. One kcal is “the calorie in our diet”, and the mol is the typical unit by which a quantity of molecules is measured by chemists (1 mol of water weighs about 18 grams).

A quantitatively correct theoretical prediction of rates of chemical reactions requires an extremely accurate description of the interaction among the atoms involved. In particular, reaching chemical accuracy for reactions on surfaces represents a tremendous challenge because it demands a simultaneous highly accurate description of two subsystems that differ greatly: molecules and metal surfaces.

The current theory allows scientists to study the reaction rate of diatomic molecules (consisting of 2 atoms) with metal surfaces using quantum mechanics for the motion of the molecule in its six degrees of freedom, while making the so-called Born Oppenheimer and static surface approximations . The precision achieved depends critically on the accuracy of the interatomic forces, as described by the so-called potential energy surface (PES). The lack of accuracy inherent in present-day density functional theory ( DFT - Density Functional Theory) the method of choice for computing PESs for molecule-surface interactions, has till now stood in the way of a quantitative description of reactions catalysed by surfaces. This international group has developed an implementation of the so-called specific reaction parameter (SRP) approach to DFT that allows a quantitative description of the molecule-surface interactions.