Obtaining Stationary States in Density Functional Theory Using Imaginary Time Propagation*

Density functional theory (DFT) is widely successful for determining electronic and structural properties of materials and molecules. In order to perform a DFT calculation, the Kohn-Sham nonlinear equations must be solved self-consistently, a task usually achieved using self-consistent field (SCF) loops. However, for large systems SCF can occasionally struggle to find the ground state. Using time-dependent DFT in imaginary time, a given starting state can be purified into its lowest-energy component with reliable monotonic convergence. With appropriate constraints, it is also possible to obtain excited states with imaginary time propagation (ITP). We compare and analyze the performance of ITP and SCF in a few difficult systems, and present a few excited state calculations performed with the method.