Bond length alternation of π-conjugated polymers predicted by the Fermi-Löwdin orbital self-interaction correction

π-conjugated polymers have found practical applications in various fields, in part due to the high delocalization of electrons through the polymer axis. For electronic structure methods (ESMs), the correct description of the delocalization level can be characterized by the bond length difference between multiple and single bonds, or bond length alternation (BLA), and is a critical test for electron correlation effects and removal of self-interaction error. The accurate theoretical determination of the BLA remains a significant challenge for traditional ESMs, such as density functional theory (DFT). Here, the BLAs of five oligomers are analyzed using the Fermi-Löwdin orbital self-interaction correction (FLOSIC) method, which was proposed as a tool to remove one-electron self-interaction from approximate DFT. The BLAs for oligomers of increasing length were extrapolated to the polymer limit and compared to DFT and MP2 results. To analyze the delocalization level predicted by each method, the Natural Bond Orbital analysis was used to quantify the deviation of the relaxed structures from the ideal Lewis structures. Our results reveal that the FLOSIC improves the calculated BLA over LSDA and PBE, but it tends to overcorrect, in line with observations for other properties.