Computing A Four-Atom Reaction

Pushing the envelope of what can be calculated by using the Schrödinger equation, a Chinese research group has completed quantum dynamical calculations of a four-atom reaction: HD + ∙ OH→ H₂O + D ∙ (Science, DOI: 10.1126/science.1205770). Such computations have been done for several three-atom systems, but expanding the approach to four atoms means accounting for twice the degrees of freedom and an exponentially greater number of quantum states. Led by Dong H. Zhang and Xueming Yang of the Chinese Academy of Sciences, the researchers developed a method that involves calculating packets of waves to map how the reaction products would scatter. Then they compared the computed map to an actual map from crossed-beam reaction experiments. The theoretical and experimental maps match up well and point to a direct hydrogen-abstraction mechanism with the atoms aligned linearly in the transition state and a strong quantum-tunneling effect for the hydrogen transfer. According to the team, the calculations took “about one month of central-processing-unit time on a computer commonly used for scientific computing,” so the approach is affordable for studying other four-atom reactions.