For several decades, density functional theory (DFT) has served as a cornerstone of electronic structure theory. Its exceptional combination of accuracy and computational efficiency has established it as the dominant method in chemistry, materials science, and condensed matter physics. Nevertheless, despite its widespread success, DFT exhibits important shortcomings, particularly for strongly correlated systems, where static and dynamic electron correlation are not treated on an equal footing.

Reduced density matrix functional theory (RDMFT) represents an appealing alternative by taking the one-body reduced density matrix, rather than the electronic density, as the fundamental variable. This more informative quantity naturally captures essential features of electronic correlation. However, RDMFT has traditionally lacked several of the key theoretical ingredients that underpin the success of Kohn–Sham DFT.

In this presentation, I will introduce a first-principles reformulation of RDMFT based on a fictitious partially interacting reference system, analogous to the Kohn–Sham framework. The corresponding effective Hamiltonian establishes a rigorous theoretical foundation and, importantly, allows for the construction of an adiabatic connection within RDMFT. We investigate the characteristics of this adiabatic connection and show how it can be employed both to study correlation effects and to inform the development of improved functionals.

By incorporating an adiabatic connection into RDMFT, we restore one of the most valuable conceptual frameworks in modern electronic structure theory within a density-matrix-based formalism. This development provides a systematic route for designing and assessing new functionals, bringing RDMFT substantially closer to becoming a practical and widely applicable alternative to conventional density functional theory.