For MEH-PPV polymers (Fig. ), examination of d exc and d ± e-h leads to the conclusion that excitations in this system can be viewed as two independent quasiparticles in the intracule and extracule coordinates of the electron/hole pair. As compared to geometric considerations, the RMS exciton size proves to be a better diagnostic for the effective size of the chromophore in a long, disordered polymer. That length scale (measured by d exc ) is sometimes longer than what might have been anticipated simply by counting conjugated bonds, due to electronic coupling between conjugatively distinct segments of the polymer. The value of d exc is effectively constant for the low-lying excited states of interest for optoelectronic applications, whereas exciton delocalization that is measured by d + e-h is found to increase with excitation energy. 151 Other statistical descriptors of an exciton include the covariance between the vectors r elec and r hole , defined as COV(r hole , r elec ) = r hole • r elecr hole • r elec .