Atomistic simulations are increasingly widespread in modern condensed matter physics, computational chemistry and materials science. They can provide insight into experimental data, elucidate mechanisms underlying real-world processes and even help to design new materials with improved properties. A wide range of approaches from quantum chemistry to machine learning interatomic potentials (MLIPs) are used, with correspondingly wide-ranging software implementations. The diversity of software packages leads to different levels of user-friendliness, interoperability, and ease of automation. 

It can be challenging to trace the origin of a calculation or ensure that the results used in a study are consistent, and it can be costly to move from one research group to another with an incompatible set of in house tools. Without shared collaborative tools, high-level algorithms such as geometry optimisation or transition state search which are in principle code independent may instead be implemented in one particular package, leaving the rest of the community to create in-house variants for their own work.This inefficiency results both in needless duplication of effort and code and the increased potential for bugs.

The Atomic Simulation Environment (ASE) is a community-driven Python package that mitigates the N² problem of maintaining pairwise interfaces between codes by providing standard data structures – principally for atomic structures (the Atoms object) and calculation methods (the Calculator object) – as well as interfaces to ca. 100 file and ca. 30 simulation codes, acting as useful "glue" for work spanning multiple packages. A 2017 paper describing ASE has attracted over 500 citations every year for the past 5 years, demonstrating the broad adoption of ASE [1].

ASE integrates methods from ab-initio and classical molecular dynamics codes. In addition, it can manipulate structures and run calculations, providing a range of generic dynamics and geometry-optimisation routines with a toolkit for the further development of these methods. New high-level algorithms are written once and immediately available to users of established atomistic codes. ASE is increasingly used as a lingua Franca for MLIP fitting and evaluation: for example the GAP, ACE, MACE, CHGNET, M3GNET, ALIGNFF and NEQUIP codes all use ASE to name but few, and the OpenKIM knowledge base of interatomic models is strongly interfaced to ASE. The existence of ASE frees the developers of new packages to focus on novel aspects, making use of existing tools for structure manipulation and dynamics. 

We are proposing to organise a CECAM workshop on ASE and its broader community of dependent packages. We think this will be a good opportunity to bring together developers and users of core ASE and other packages in its ecosystem. This workshop is timely because ASE has grown rapidly in recent years, meaning that maintaining it is becoming a challenge for the core development team. At the same time, ASE has effectively become a critical piece of research infrastructure for an increasingly broad community of users and developers. An effective way ahead has been identified through the establishment, promotion and curation of an interoperable ecosystem of packages rather than a monolithic code base. 24 such packages have already been identified; their authors are represented amongst our proposed invited speakers. It is now critical to assemble a team of developers and end users to consider how to make this transition smoothly and sustainably.

Challenges to be addressed at the workshop will include:

• What are the dominant current use cases of ASE? How will this change in the next 5 years?
• What functionality should the core ASE include?
• Are the current Atoms and Calculator interfaces sufficient, or do they need to be generalised? How should they be serialised to/from files?
• What can be moved from the core code to plugins?
• How can the promotion of a distributed ecosystem of plugin file-formats and calculators be catalysed?
• How can the development of the ASE ecosystem remain sustainable?
• Does a community governance model for ASE need to be established?