Assessing the role of Hartree-Fock exchange, correlation energy and long range corrections in evaluating ionization potential, and electron affinity in density functional theory

Abstract

Accurate determination of ionization potentials (IPs), electron affinities (EAs), fundamental gaps (FGs), and HOMO, LUMO energy levels of organic molecules play an important role in modeling and predicting the efficiencies of organic photovoltaics, OLEDs etc. In this work, we investigate the effects of Hartree Fock (HF) Exchange, correlation energy, and long range corrections in predicting IP and EA in Hybrid Functionals. We observe increase in percentage of HF exchange results in increase of IPs and decrease in EAs. Contrary to the general expectations inclusion of both HF exchange and correlation energy (from the second order perturbation theory MP2) leads to poor prediction. Range separated Hybrid Functionals are found to be more reliable among various DFT Functionals investigated. DFT Functionals predict accurate IPs whereas post HF methods predict accurate EAs. © 2017 Wiley Periodicals, Inc.

Impact of non‐empirically tuning the range‐separation parameter of long‐range corrected hybrid functionals on ionization potentials, electron affinities, and fundamental gaps

“…Hybrid functionals generally outperform pure density functionals (e.g., those based on the local density approximation and generalized gradient approximations [GGAs]) in evaluating IPs, EAs, and FGs . Our recent investigations concluded that among the various types of XC functionals available (e.g., global hybrid, long‐range corrected (LC) hybrid, and double‐hybrid functionals), the performance of LC hybrid functionals is superior and consistent for the IPs, EAs, and FGs of neutral molecules . Similar conclusions were also made in earlier studies .…”

“…These materials are gradually replacing the traditional materials (e.g., their inorganic counterparts) in different fields that have been dominating the markets over the past few decades. The function and performance of these materials depends heavily on their optoelectronic properties, including the vertical ionization potentials (IPs), vertical electron affinities (EAs), fundamental gaps (FGs), optical gaps, and so forth . For example, organic light emitting diodes contain several layers of different stacked organic films, and there exists an energy barrier to the flow of charge between these layers, therefore, the knowledge of transport energy levels is required along with IPs, EAs, and FGs that can help in optimizing and designing the devices with higher efficiencies.…”

“…Hence, we perform a benchmarking study to evaluate the significance of tuning scheme in improving the accuracy of IPs, EAs, and FGs. We mainly adopt LC‐ωPBE (i.e., a popular LC hybrid functional) to evaluate the IPs, EAs, and FGs of 30 organic molecules of different types, employing both the default and tuned range‐separation parameters, where the reference data were obtained from the highly accurate CCSD(T) calculations in our previous study . In this manuscript, we also examine the reliability of tuning scheme with two other popular LC hybrid functionals, LC‐BLYP and ωB97X‐D, for comparisons.…”

Limitations of Global Hybrids in Predicting the Geometries and Torsional Energy Barriers of Dimeric Systems and the Role of Hartree Fock and DFT Exchange

“…In order to obtain quantitatively accurate results for benchmarking the DFT methods, torsional energies are evaluated employing CCSD/ATZ with the optimized geometries of CCSD/ADZ at 90 0 , 0 0 , and 180 0 , respectively. (Note: the dihedral angle of dimer is fixed and the remaining geometry is allowed to optimize with CCSD/ADZ @ 0 0 , 90 0 , and 180 0 ) Larger basis set ATZ with CCSD provides accurate relative energies, which are closer to the complete basis set limit . Torsional energy barriers employing CCSD/ADZ vary from CCSD/ATZ (Supporting Information Tables S1, S2, S3, and S4).…”

“…(Note: the dihedral angle of dimer is fixed and the remaining geometry is allowed to optimize with CCSD/ADZ @ 0 0 , 90 0 , and 180 0 ) Larger basis set ATZ with CCSD provides accurate relative energies, which are closer to the complete basis set limit. [31] Torsional energy barriers employing CCSD/ADZ vary from CCSD/ATZ (Supporting Information Tables S1, S2, S3, and S4). Ab-initio methods are relatively more sensitive to the basis set compared to DFT methods (≤0.13 kcal/mol on dimeric systems).…”