https://sangjun-sim.github.io/A minimal, responsive and feature-rich Jekyll theme for technical writing. 2026-08-14T00:14:24+00:00 Sangjun Sim https://sangjun-sim.github.io/ Jekyll © 2026 Sangjun Sim /assets/img/favicons/favicon.ico /assets/img/favicons/favicon-96x96.png 2026-08-14T00:00:32+00:00 2026-08-14T00:00:32+00:00 https://sangjun-sim.github.io/posts/essay-seneca/ Sangjun Sim

On the Happy Life Seneca defines the happy life as possessing a soul that is free, upright, and steadfast — one liberated from all fear and desire (p. 63). For him, happiness is not found in outwardly splendid places, but in what lies close at hand around us. To attain happiness (or the highest good), he argues that one must (1) find one’s own destination and the right path to reach it, rather...

2026-08-10T00:00:30+00:00 2026-08-10T00:00:30+00:00 https://sangjun-sim.github.io/posts/symmetry-kitaev/ Sangjun Sim

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2026-08-07T00:00:00+00:00 2026-08-10T02:46:22+00:00 https://sangjun-sim.github.io/posts/cond-matter-paircol/ Sangjun Sim

A short note on pair-correlation function The pair-correlation function is the concept in the many-body system that can be applied to understand the physical origin of the exchange-correlation energy. This function corresponds to the “correlation function” in the statistics. The pair-correlation function and its multiplication to the charge density at $\mathbf{r}’$, $n(\mathbf{r}’)$ is written...

2026-08-03T00:00:20+00:00 2026-08-03T00:00:20+00:00 https://sangjun-sim.github.io/posts/cond-matter-pseudopot/ Sangjun Sim

A short note on pseudopotential Pseudopotentials are artificial potentials created in atomic configurations that smooth out potentials in the core region. This method assumes that the core state is insensitive to the environment and attempts to describe the core electrons with smooth potentials. This concept is based on the idea of the Orthogonal Plane Wave (OPW) method [Herring, Callaway], wh...

2026-07-31T08:00:00+00:00 2026-07-31T08:00:00+00:00 https://sangjun-sim.github.io/posts/cond-matter-janak/ Sangjun Sim

A short note on Janak theorem The total energy derived by the Kohn-Sham (KS) equation, $E[n]$ inherently does not have any physical meaning. In the Hartree-Fock approximation, the energy required to subtract or add an electron from a single orbital is the eigenvalue, as Koopman’s theorem states. However, in the KS scheme, such energy is not the eigenvalue. Instead, the KS eigenvalue truly beco...