Spring School Computational Tools: Day 4 – Gaussian

After having focused on solids during the previous two days of our spring school, using either VASP (Tuesday) or ABINIT (Wednesday), today’s focus goes to molecules, and we turn our attention to the Gaussian code.

Dietmar Hertsen introduced us into the Gaussian code, and immediately showed us why this code was included in our spring school set: it is the most popular code (according to google). He also explained this code is so popular (among chemists) because it can do a lot of the chemistry chemists are interested in, and because of the simplicity of the input files for small molecules. After pointing out the empty lines quirks of Gaussian it was time to introduce some of the possible editors to use with Gaussian. The remainder of his lecture, Dietmar showed us how simple (typical) Gaussian calculations are run, pointing out interesting aspects of the workflow, and the fun of watching vibrations in molden. He ended his lecture giving us some tips and tricks for the investigation of transition states, and the study of chemical reactions, as a mental preparation for the first hands-on session which followed after the coffee-break.

Lecturers for the Gaussian code.

Lecturers for the Gaussian code: Dietmar Hertsen (left) introducing the basics of the code, while Patrick Bultinck (right) discuses more advanced wave function techniques in more detail.

In the afternoon it was time to take out the big guns. Prof. Patrick Bultinck, of the Ghent Quantum Chemistry Group, was so kind to provide the advanced session. In this session, we were reminded, after two days of using the density as a central property, that wave functions are the only way to obtain perfect results. Unfortunately, practical limitations hamper the application to the systems of interest from a practical physical point of view. Patrick, being a quantum chemist to the bone, at several points stepped away from his slides and showed on the blackboard how several approximations to full configuration interaction (full-CI) can be obtained. He also made us aware of the caveats of such approaches; Such as size consistency and basis(-type) dependence for truncated CI, and noted that although CASSCF is a powerful method (albeit not for the fainthearted), it is somewhat a black art that should be used with caution. As such, CASSCF was not included in the advanced hands-on sessions guided by Dr. Sofie Van Damme (but who knows what may happen in a future edition of this spring school).

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