where  is the fine-structure constant, me is the electron mass, c is the speed of light. H (E -ha) is the Heaviside step function, ensuring that the emission energy E is smaller than the excitation energy ha. Np is the total number of geometries Rn in the ensemble. E10 is the transition energy between T1 and S0 computed for each geometry Rn. 10 f is the oscillator strength between the spin-perturbed 1 T and 0 S states for each geometry. Its calculation is discussed later in this section. ws is a normalized sharp line shape (a Gaussian function, for instance) centered at E10 and with width .