The literature to date contains only a few references to the
calculation of excited states within the QMC formalism. The most
notable are those of Mitas and Martin[51, 76], who
calculated the electronic energy gap for a compressed molecular
nitrogen solid and carbon and compared them with LDA and Hartree-Fock
estimations, Knorr and Godby[77] who performed DMC
calculations on semiconductor wires, Fahy et
al.[78] who calculated the quasiparticle
energies within a single-mode approximation in diamond, and Engel et
al.[79], who compared the quasiparticle bands in a
two-dimensional crystal calculated using GW and QMC. QMC
calculations of excited states via indirect
methods[80, 81, 82], such as
extracting excitation energies from the exponential decay curves in
DMC, are discussed in section 
.