[AZ.08.03.2025]
6-31G* (p2p) is the minimal reasonable basis for conjugated molecules. Accurate energies can be obtained with Def2-TZVP (a3p) but at the expense of doubling the basis set size (stilbene, P3HT). As a trade-off, an expansion of 6-31G* can be used such as 6-311G*+ (p3pa) but benchmarking is needed per system. Starting from potassium, 6-31G* becomes unreliable so that a composite basis should be used: 6-31G* for "organic atoms" and Def2-TZVP for transition metals and heavier elements (Ni-BTA). For plane waves with PAW pseudopotentials in VASP the minimal cutoff is about 400 eV [VASP]. However, the unit cell optimization in VASP requires much larger cutoffs of up to 900 eV (BMCOS1 benchmark).