Which is the best DFT functional and basis set for polymer structure optimisation? Looking for frequency calculations and HOMO-LUMO studies.
When it comes to polymers, you are in a field in which several things compete, especially "realistic system size" vs. "calculation costs".
Generally, high level DFT with hybrid or meta-GGA functionals would be desirable, but a polymer with a realistic length would make the calculation to long and therefore expensive. Therefore you probably will have to settle with a GGA-level functional ("the" classic would be BP86, but check out literature from yoiur field) and a double or triple zeta valence level basis set (e.g. def2-SVP or def2-TZVP).
The length and size issue could be overcome by a supercell approach, if the monomers are not too complicated, see e.g.
Article A DFT study of the structural and electronic properties of p...
However, when it comes to calculating vibrational spectra, for polymers you run into the issue that you can have a lot of conformers. Therefore a molecular dynamics simulation in combination with FTTCF might be a better option, see e.g.
Article Vibrational Raman Spectra from the Self-Consistent Charge De...
Whether you can perform a DFT-MD or whether you need a semiempiric one depends on how much computational power you have available and how complicated your polymers are.
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