Semiclassical Correlation in Time-Dependent Density Matrix Functional Theory

The impact of time-dependent density functional theory (TDDFT) [1, 2] on calculations of excitation spectra and response in atoms, molecules, and solids is evident in its increasing use. In such applications a weak perturbation is applied to the system beginning in its ground state, and usually the exchange-correlation (xc) effects are treated with a ground-state approximation. Generally the results are usefully accurate, but specific cases (e.g. charge transfer excitations, optical response of solids, etc) require improved approximations undergoing intense research.