Tuning range-separated DFT functionals for modeling the peak absorption of MEH-PPV polymer in various solvents

Density Functional Theory (DFT) and time-dependent Density Functional Theory (TD-DFT) are powerful tools for modeling orbital energy in conjugated molecules. Range Separated (RS) functionals vary the percentage of Hartree-Fock and DFT exchange for long-range and short-range interactions and are used in cases involving charge transfer excitation. The accuracy of the computed results using RS functionals depends on the range-separation parameter (omega) whose optimal values are not always known. Conjugated polymers add additional difficulty because the length of the polymer can vary. In this study, accurate values of omega for three lengths of MEH-PPV polymer (trimer, tetramer, and pentamer) in five different solvents (chloroform, chlorobenzene, xylene, Tetrahydrofuran, and dichloromethane) are reported using the RS functionals wB97XD and CAM-B3LYP. Range separation parameters are predicted and used for longer polymer chains. The differences in the omega for different solvents is statistically significant and gives insight into the polymer/solvent interaction.