Neutral band gap of carbon by quantum Monte Carlo methods

We present a method of calculating the energy gap charge-neutral excitation using only ground-state calculations. report Quantum Monte Carlo calculations Γ→ Γ and → X particle-hole energies in diamond carbon. analyze finite-size effect find same 1/L decay rate as that charged excitation, where L is linear extension supercell. This slow attributed to delocalized nature supercells too small accommodate excitonic binding effects. At larger system sizes, apparent crosses over 1/L3 behavior. Estimation scale exciton can be used correct effects neutral gaps.