ELSI – ELectronic Structure Infrastructure provides and enhances scalable, open-source software library solutions for electronic structure calculations in materials science, condensed matter physics, chemistry, molecular biochemistry, and many other fields. ELSI focuses on methods that solve or circumvent eigenvalue problems in electronic structure theory. The ELSI infrastructure should also be useful for other challenging eigenvalue problems.

ELSI deals with the Kohn–Sham eigenvalue problem, which is central to Kohn–Sham density-functional theory, one of the most widely used methods in electronic structure. This problem is often the bottleneck in large scale calculations by high-performance computation and many different algorithms and strategies exist to tackle it. ELSI acts as a unified software interface to access different algorithms and their corresponding implementations. This greatly simplifies the implementation and optimal use of the different strategies.