  Density functional theory (DFT), the most widely adopted method in modern
computational chemistry, fails to describe accurately the electronic structure
of strongly correlated systems. Here we show that DFT can be formally and
practically transformed into a one-electron reduced-density-matrix (1-RDM)
functional theory, which can address the limitations of DFT while retaining
favorable computational scaling compared to wavefunction-based approaches. In
addition to relaxing the idempotency restriction on the 1-RDM in the kinetic
energy term, we add a quadratic 1-RDM-based term to DFT's density-based
exchange-correlation functional. Our approach, which we implement by quadratic
semidefinite programming at DFT's computational scaling of $O(r^{3})$, yields
substantial improvements over traditional DFT in the description of static
correlation in chemical structures and processes such as singlet biradicals and
bond dissociations.

Chemical Physics;Computational Physics;Quantum Physics