Excitation energies can be found by taking the difference between
the ground state energy of systems with different numbers of electrons.
For example, a gap energy 
can be written as
where 
is the energy to add an electron (the electron
affinity
) and 
is the energy to remove an electron (the ionization potential).
To obtain an estimate of the electron(hole) energies, 
, within a QMC calculation, all that is
required is to include (remove) the corresponding conduction (valence)
orbital in the Slater Determinant part of the trial wavefunction given
in Eq.(
). The 
-point at which the
electron(hole) energy is calculated, is determined by the
-value of this conduction (valence) orbital.
