Excitations and quasi-excitations in jellium, and excitations in quantum materials with linear response time dependent density functional theory*

In the framework of linear response time-dependent density functional theory (LR-TDDFT), the exact exchange-correlation (xc) kernel determines the ground-state energy, excited-state energies, lifetimes, and the time-dependent linear density response of any homogeneous many-electron system [1,2]. 

The dielectric function in principle can encompass xc effects relevant to describe low-density physics in the homogeneous electron gas. Beside collective plasmon excitations, the dielectric function can reveal collective electron-hole excitations, often dubbed “ghost excitons” [3]. The physics in the low-density regime is rich, as exemplified by a static charge-density wave that was recently found with the MCP07 kernel and was shown to be associated with softening of the plasmon mode.

In this talk I will give a detailed view of xc model kernels for excitations of various kinds in the low-density homogeneous electron gas. Furthermore, I will also give some details of an ongoing effort to extend the applicability of model xc kernels to excitations in real quantum materials exemplified byTiSe₂ [4].

[1] A. Ruzsinszky, N.K. Nepal, J.M. Pitarke, J.P. Perdew, Phys. Rev. B 101, 245135 (2020)   

[2] J.P. Perdew, A. Ruzsinszky, J. Sun, N. K. Nepal, and A. D. Kaplan, Proceedings of the National Academy of Sciences of the United States of America, 118 (4) (2021)

[3] A. D. Kaplan, A. Ruzsinszky, J. Chem. Theo. Comp. submitted, arXiv preprint arXiv:2308.15392.

[4] H. Tang A. Ruzsinszky, in preparation.