# Density functional theory calculations of large systems: Interplay between fragments, observables, and computational complexity

I am happy to announce a new publication available in WIREs Computational Molecular Science entitled Density functional theory calculations of large systems: Interplay between fragments, observables, and computational complexity.

For many years now, it has been possible to use our BigDFT code to perform DFT calculations on systems composed of many thousands of atoms. As someone who is primarily interested in the computational side of things, this felt like a clear win. Nonetheless, for a code to be really useful it has to prove its worth in actual studies. For this review, we wanted to look at what kind of real DFT studies are being performed on large systems in hopes that we would gain insight for future research directions.

Surprisingly, there are fewer application papers than one would expect given the capabilities of existing DFT codes. One challenge is that larger systems are more complex, so it is even harder to go from a single point calculation to actual rational insight. Therefore calculations on large systems must be conceived quite differently than when working on small, model systems.

To address this issues, we defined a framework of study based on the concept of a "quasiobservable". In a standard quantum mechanical investigation, we treat the full system as one coherent unit, and compute observables of the entire system. At a large scale, however, it becomes possible to break down a system into a set of well defined fragments, which are mutually entangled. Then, we can define quasiobservables which are associated with each fragment (or sets of fragments). These quasiobservables become localized descriptors of a system which can be more easily turned into insight about emergent properties.

While the way we described our framework is new, we found many excellent studies which are already using such an approach. I hope that our review can highlight these new types of studies and that it will inspire you towards new research projects.