<h3>Title</h3>
<h3>Modelling optical properties of f-centre defects: A generalised KS semicanonical projected RPA perspective</h3>
<p><strong><span style="text-decoration: underline;">Abstract</span> :</strong></p>
<p style="text-align: justify;">Defects can dominate the electronic and optical properties in materials, thus play a vital role in the performance of many technological applications. Among various defects, our primary focus is on f-centre defects which can act as a strong local-reduction site in the chemical processes. The experimental measurement of such defects are usually hampered by their low concentration. Therefore, theoretical studies of defect sites are of major importance for a deeper understanding of how defects might work. Theoretical studies based on periodic supercell model can be a method of choice, but encounters higher defect concentration. It is also computationally demanding using quantum chemistry (QC) methods. On the other hand, embedded cluster model is capable of describing isolated point defects along with the highlevel QC methods. In this direction, we present periodic electrostatic embedding cluster model (PEECM) in which the quantum mechanical part is treated with the recently developed generalised Kohn-Sham semicanonical projected RPA (GKS-spRPA) theory. A detailed systematic assessment of properties towards the bulk limit is presented here on a representative f-center LiF. In all instances, these results are excellent agreement with the available theoretical and experimental results. At the end, I will discuss about the importance of surface defects as well as major challenges to model such defect within the PEECM-GKS-spRPA framework.</p>
<p style="text-align: justify;">In the second part of my presentation, I will discuss about metastable systems which are observed in radioactive decays, dissociative electron attachment, stark ionisation, even in biomolecules like DNA base pairs, etc. These states are characterised by complex energy, the real part of which is the resonance energy and the imaginary part is associated with its lifetime. The GKS-spRPA can be a method of choice for such problems, but it requires the analytical continuation of real energy to complex domain. Here, a non-Hermitian analogue of adiabatic connection is derived. Further, it has been shown that it is same as the Hermitian approach as long as the non-Hermitian part is symmetric and local. Finally, the ongoing work and future direction is discussed.</p>