This section describes two approaches for computing these quantities. The first is the nuclear ensemble approach (NEA), in which the emission band is given by a sum of transitions from many different nuclear geometries, representing the nuclear wavefunction density in the source states. (The source states are those the molecule initially populates before the emission.) The second is the vertical convolution approach (VCA) -a widely used spectrum simulation technique based on the vertical transitions at the equilibrium geometries convoluted with arbitrary band shape functions.