The Einstein solid models a crystal as N independent quantum harmonic oscillators all sharing the same frequency ω_E. Each oscillator has energy levels E_n = ℏω(n + ½), populated according to the Bose-Einstein distribution ⟨n⟩ = 1/(e^(ℏω/kT) − 1). The heat capacity C_V = 3Nk(ℏω/kT)² e^(ℏω/kT)/(e^(ℏω/kT)−1)² vanishes exponentially at low T (explaining the Dulong-Petit failure) and recovers the classical 3Nk at high T. This 1907 model was among the first successful quantum predictions about macroscopic materials.