Modeling for HOMO and LUMO for organic molecule

[rishikanta_m]

Dear VASP Forum,

I am currently working on energy level and band bending analysis for a semiconductor-organic molecule interface using VASP. However, I am facing an issue where the HOMO and LUMO energy levels of the organic molecule are significantly deeper than expected, resulting in poor alignment with the semiconductor's band edges. This misalignment is affecting the expected electronic interactions at the interface.

Could you please suggest possible approaches or techniques that could be employed within VASP to shift the energy levels of the organic molecule upwards, relative to the semiconductor, to improve the alignment?

[martin.schlipf]

There are many different techniques to change the level alignment in VASP. The most common approach would be to investigate whether different xc functionals can yield better results. This depends strongly on the physics that cause the misalignment. Typical choices would be metaGGAs (e.g. SCAN or MBJ) or hybrid functionals (e.g. HSE or PBE0).

A more expensive solution would be to compute GW or other many-body corrections. Keep in mind that this is orders of magnitude more demanding than local functionals.

Another possibility is to employ DFT+U. This adds an on-site Coulomb correction that changes the alignment of the corresponding bands. The computational cost is comparable to a local functional. The disadvantage of this approach is that the U chosen is typically an empirical parameter. There are ways to determine U ab initio but this adds more complexity.

[rishikanta_m]

Thank you for your suggestions, Martin.

The calculations involve a semiconductor slab (approximately 3 nm) and an organic molecule, with a vacuum present as well. Hybrid functionals do not seem to be a suitable option. DFT-1/2 provides a good band gap for the slab under quantum confinement. Is it feasible to modify the potentials of the atoms comprising the molecule to correct its band gap and achieve proper alignment with the slab?

[martin.schlipf]

What is DFT-1/2?

Regarding changing the potential, there is a way but that involves some coding. Assuming you have a recent version of VASP there is an ADD_TO_EXTERNAL_POTENTIAL routine in pot.F. If you call this routine at some point before with an appropriately constructed external potential, VASP should compute the correct ground state given this external potential.

We plan to make this kind of features more user friendly in the future but at the moment you need to be able to read and write Fortran code to do that.