Time-dependent DFT

In this section, the time-dependent density functional theory implementation in BAND is described. How to do a response calculation in BAND (which input keys to use), can be found in the list of keywords, see the key Response. The TDDFT module enables the calculation of real and imaginary parts of the material property tensor χ_e(ω) called the electric susceptibility, and the macroscopic dielectric function ε_e(ω) These are mutually related,

In general χ_e(ω) and ε_e(ω) are tensors, which, however, simplify to scalars in isotropic systems. The above formula is valid in the case of insulators and semiconductors, where the bands are either fully occupied or fully unoccupied. It is defined as the interband part of the dielectric function and it is due to transitions from occupied bands to unoccupied bands. Also in the metallic case similar expressions can be found, the main difference is that there now is also an intraband contribution. Some examples are available in the $ADFHOME/examples/band directory and are discussed in the Examples document.