Exciton screening in C$_{60}$ and PTCDA complexes. TDDFT calculations with GGA and hybrid functionals

TL;DR

This study uses TDDFT with various functionals to analyze exciton energies in C60 and PTCDA complexes, highlighting the impact of exchange-correlation choices on charge-transfer excitons.

Contribution

It compares the effectiveness of GGA and hybrid functionals in predicting exciton energies, especially for charge-transfer states, revealing limitations of hybrid functionals at long-range.

Findings

Hybrid functionals improve short-range exciton energy accuracy.

Accuracy of hybrid functionals decreases for long-range excitons.

PBE functional outperforms hybrids when exciton radius approaches screening length.

Abstract

Photoabsorption in the low-energy region for C$_{60}$ and PTCDA molecular complexes is studied within linear response TDDFT. For the PBE, B3LYP and HSE exchange-correlation (xc) functionals the dependence of the accuracy of the exciton energy on the electron-hole separation is analyzed. Particular attention is paid to the charge-transfer (CT) excitons. The inclusion of non-local exchange using hybrid functionals increases the accuracy of calculations for short-range excitons, however, the accuracy of hybrid functionals decreases significantly for long-range excitons. Moreover, as the exciton radius approaches the "screening length"\ , the simpler PBE functional gives more accurate excitonic energies than the mentioned hybrid functionals.