The Different Levels of Quantum: A Methodological-Philosophical Portrait of Computational Quantum Chemistry

In silico and in vitro experimental chemistry do have many similarities in terms of following the scientific method, but they also have epistemological differences. Maybe the most striking difference is that wet chemistry, in terms of physical effects, is all-inclusive; it is not possible to remove relativistic effects or the zero-point energy (to give a couple of examples) from the molecules in a real-life flask. On the contrary, computational chemistry requires a conscious and careful decision on whether these specific effects should be included in the virtual experiment. Omit them, and the chemistry will not be realistic; incorporate unnecessary effects, and the job may be intractable. In this perspective, we will describe one direction to add quantum effects, depending on the reach and requirements of the studied question. From the electronic to the nuclear wave function and beyond, the addition of deeper and deeper quantum effects must be sequentially and judiciously nested as a matryoshka doll.