On the calculation of general response properties in subsystem density functional theory

A recently developed subsystem formalism within time-dependent density functional theory for excitation energies [J. Chem. Phys. 126, 134116 (2007)] is extended to the calculation of general response properties such as frequency-dependent polarizabilities or optical rotatory dispersion. Furthermore, explicit expressions for oscillator and rotatory strengths are presented. Polarizabilities and optical rotation tensors of the combined system are obtained as the sum of subsystem properties, so that a natural partitioning of the polarizability or the optical rotation is facilitated. This is of great advantage for the analysis of environmental effects on molecular response properties as it allows one to distinguish changes in the ground-state electronic structure of the subsystems from cooperative excited-state effects. A computational scheme that implements the formalism is presented together with applications highlighting the differences between embedding effects included in the subsystem potential and environmental response effects. The method avoids basis-set superposition errors in the calculation of response properties of large molecules, which occur in supermolecular calculations. Possible approximations to the full response algorithm are discussed.