In LDA, the exchange–correlation energy is typically separated into the exchange part and the correlation part: ε XC = ε X + ε C. The exchange part is called the Dirac (or sometimes Slater) exchange, which takes the form ε X ∝ n. There are, however, many mathematical forms for the correlation part. Highly accurate formulae for the correlation energy density ε C (n ↑, n ↓) have been constructed from quantum Monte Carlo simulations of jellium. A simple first-principles correlation functional has been recently proposed as well. Although unrelated to the Monte Carlo simulation, the two variants provide comparable accuracy.