Our laboratory’s main theme is the first-principles quantum mechanical calculations of light-matter interactions or full ab initio description of materials’ quantum states under a strong or weak light field. To materialize this vision, we have worked on the electron-correlation part of the time-dependent density functional theory(DFT) and explored the limitation and capability of DFT-based methods.
Since Schrödinger’s and Dirac’s formulation of quantum mechanics at around the late 1920s, diverse branches of theories have developed to deal with quantum mechanics of condensed matters in a way as ab initio as possible. The relativistic or non-relativistic Hamiltonian consists of two-body interactions of every pair of many bodies, which gives rise to enormous complexity in the ab initio treatment. One of the most common approaches is the density functional theory(DFT), in which an effective one-body Hamiltonian is solved self-consistently, and various hierarchical treatments have been introduced to overcome the limitations of such an effective one-body theory. While these methods have been remarkably successful for the ground state or the excited state within the linear response regime, the ab initio description for the non-linear regime of the excited state has remained challenging. On the other hand, the experimental side’s community is rapidly growing toward non-equilibrium or transient phenomena, mainly through the femtosecond pump and probe techniques. Such measurements include perturbations, such as photons or electrons colliding with a condensed matter, and the transient phenomena at the boundary of static phases of multiferroic structures.