CECAM has held many meetings on the "Electronic Structure Library" (ESL) [1], which have mostly been focused on solid state electronic structure programs. Recent meetings have mainly consisted of presentations of individual software packages, most of which are monolithic in character. There have hardly been any attempts to discuss refactoring codes to afford new synergies by identifying and introducing novel reusable libraries.

Moreover, there has been little discussion on software and methods employed in quantum chemistry, and how to extract possible synergies for quantum chemistry methods. Although standardization of quantum chemistry methods has been attempted in the past—for instance, there are rumors about a meeting organized at Argonne National Laboratory in the late 1980s or early 1990s with the aim to standardizing aspects of quantum chemistry programs—it is clear that the time has now become ripe for a new attempt. While distributing software was difficult in the past, handled by sending magnetic tapes or floppies in the mail, nowadays libraries are straightforward to develop and distribute in a decentralized manner over git [2], for instance. Contrary to the situation even a decade ago, we also have now a rich ecosystem of open source quantum chemistry programs [3], which afford an advantageous development platform for open source reusable libraries: open source projects will likely be more enthusiastic about relying on new reusable open source libraries than established proprietary packages, which tend to have and prefer a monolithic design.

There already exists a flourishing ecosystem of reusable libraries for various purposes in quantum chemistry, such as libraries for geometry optimization (for example: GeomeTRIC [4] and OptKing), Gaussian integrals (libcint [5], libint and libintX [6], etc.), calculations with the algebraic diagrammatic constructor [7] (adcc) or density matrix renormalization group (DMRG) [8], and so on. A challenge in this diverse landscape is even to be aware of the multitude of projects that are on going around the world. Meetings with a limited focus, such as those on a single library such as the CECAM ESL, or one single task like tensor contractions can be invaluable in advancing software for those purposes, yet more general meetings are also necessary to capture the diversity of the field. Moreover, meetings limited to primarily a European (CECAM) or a United States (MolSSI) audience do not suffice to thoroughly capture its breadth, and an international meeting would be highly beneficial to catalyze and fertilize the organic developments in the greater computational chemistry software ecosystem.