Kohn's theorem is a principle in quantum mechanics that states that the ground state energy of a many-electron system can be determined without considering electron-electron interactions. This theorem is particularly relevant in the context of density functional theory (DFT), where it simplifies calculations by allowing researchers to focus on the electron density rather than the complex interactions between individual electrons. The theorem is named after Walter Kohn, who was awarded the Nobel Prize in Chemistry in 1998 for his contributions to the development of DFT. Kohn's theorem helps in understanding the behavior of electrons in materials and is widely used in computational chemistry and solid-state physics.