We start with an atomistic model of a supramolecular assembly of P3HT chains (schematic presentation is shown in Supplemental Figure ), each exactly 30 thiophene monomer units long, that mimics the fibers prepared by living crystallization self-assembly . We resort to a Frenkel-Holstein type mixed quantum-classical model including a multicentric transition density description of the Coulombic interchain excitonic interactions ( ) and accounting for the coupling to a leading high-frequency carbon-carbon stretching/breathing vibrational mode per chain ( )