Molecular orbital calculations in the footsteps of Coulson

Today I ran a workshop for some of our final year Chemistry students with the dual aims of giving them experience of running the Gaussian code and repeating Coulson’s seminal calculations from 1937. Added to that I hope it helped them make some more sense of my lectures on SCF calculations!

As a ‘lab’ exercise it was administratively straightforward: the calculations were easily completed in an hour, and the write-up could mostly be done at the same time (with good organisation). Submission was by a word document online, and I know quite a few of the class submitted their reports within a few hours of completing the workshop.

As for the science, the workshop involved using Gaussian to optimise the geometry of molecular hydrogen. Two minimal basis sets were used (STO-3G and STO-6G), and two split valence basis sets (3-21G and 6-31G). The results clearly showed the superiority of the more complex basis sets, but it is worth mentioning that in 1937 (before the advent of computers and Pople’s use of Gaussian functions and development of the Gaussian program) Coulson managed to get superior results compared to those obtained with minimal basis sets using the Gaussian program! But it has to be said that it would be challenging to extend his calculations to larger molecules.

Overall I was happy with the way everything went in the workshop. I have some ideas for minor tweaks for next year, but nothing major. I am more concerned with how the Quantum Chemistry lecture material has been received, but I’ll have to wait for the exam and module questionnaire to find out!

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