1. Could you explain the significance of your article to the non-specialist? 

We calculate from first principles (ab initio) the first modern "potential energy surface," or force field, for the interaction between two very fundamental molecules: hydrogen and carbon dioxide. The surface is four-dimensional, since it involves the separation of the molecules and the three angles which describe their relative orientation. Its accuracy is verified by stringent comparison with highly resolved infrared spectra of the weakly-bound H₂-CO₂ van der Waals complex. As a result, the previously mysterious H₂-CO₂ spectrum involving the ortho spin modification of H₂ is now understood.

 

2. What has motivated you to conduct this work? 

This paper is an example of "internet science". When the Chinese researchers contacted the Canadian researcher (whom they have never met) by e-mail, they were puzzled because their theoretical results agreed beautifully with his published experimental spectrum of para H₂-CO₂, but seemed not to agree with that of ortho H₂-CO₂. In the course of resolving this problem, it was realized that simple symmetry considerations explain why the ortho H₂-CO₂ and para H₂-CO₂ spectra are so different. 

 

3. Where do you see this work developing in the future? 

Accurate knowledge of the H₂-CO₂ interaction potential will be important for the interpretation of experiments (now underway) to measure the infrared spectra of small hydrogen clusters containing a single CO₂ "probe" molecule. These results on (H₂)_N-CO₂ clusters with N = 1 - 20 should help to answer the question as to whether small para H₂ clusters exhibit superfluid type behaviour similar to that shown by ⁴He clusters in the same size range. Para H₂, with resultant nuclear spin I  = 0 and rotational angular momentum J = 0 is a (composite) boson like ⁴He.

 

4. Are there any particular challenges facing future research in this area? 

Ab initio molecular structure calculations are rapidly improving thanks to increasing computer power and more sophisticated theories. In spite of this progress, obtaining accurate results for weak (van der Waals type) interactions remains especially challenging. The ability to provide accurate ab initio calculations of weak interactions is increasingly important because of applications to larger systems relevant to molecular biology (e.g. protein folding). It is therefore vital to be able to test and verify weak interaction calculations by comparison with experiment, and high resolution spectra of van der Waals complexes provide the most direct, precise, and unambiguous means to accomplish this comparison.