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Abstract
A new molecular dataset called HAB79 is introduced to provide ab initio reference values for electronic couplings (transfer integrals) and to benchmark density functional theory (DFT) and density functional tight-binding (DFTB) calculations. The HAB79 dataset is composed of 79 planar heterocyclic polyaromatic hydrocarbon molecules frequently encountered in organic (opto)electronics, arranged to 921 structurally diverse dimer configurations. We show that CASSCF/NEVPT2 with a minimal active space provides a robust reference method that can be applied to the relatively large molecules of the dataset. Electronic couplings are largest for cofacial dimers, in particular, sulfur-containing polyaromatic hydrocarbons, with values in excess of 0.5eV, followed by parallel displaced cofacial dimers. V-shaped dimer motifs, often encountered in the herringbone layers of organic crystals, exhibit medium-sized couplings, whereas T-shaped dimers have the lowest couplings. DFT values obtained from the projector operator-based diabatization (POD) method are initially benchmarked against the smaller databases HAB11 (HAB7-) and found to systematically improve when climbing Jacob's ladder, giving mean relative unsigned errors (MRUEs) of 27.7% (26.3%) for the generalized gradient approximation (GGA) functional BLYP, 20.7% (15.8%) for hybrid functional B3LYP, and 5.2% (7.5%) for the long-range corrected hybrid functional omega-B97X. Cost-effective POD in combination with a GGA functional and very efficient DFTB calculations on the dimers of the HAB79 database give a good linear correlation with the CASSCF/NEVPT2 reference data, which, after scaling with a multiplicative constant, gives reasonably small MRUEs of 17.9% and 40.1%, respectively, bearing in mind that couplings in HAB79 vary over 4 orders of magnitude. The ab initio reference data reported here are expected to be useful for benchmarking other DFT or semi-empirical approaches for electronic coupling calculations.
Highlights
density functional theory (DFT) values obtained from the projector operator-based diabatization (POD) method are initially benchmarked against the smaller databases HAB11 (HAB7-) and found to systematically improve when climbing Jacob’s ladder, giving mean relative unsigned errors (MRUEs) of 27.7% (26.3%) for the generalized gradient approximation (GGA) functional BLYP, 20.7% (15.8%) for hybrid functional B3LYP, and 5.2% (7.5%) for the long-range corrected hybrid functional omega-B97X
In our previous scitation.org/journal/jcp study,19 we showed that MRCI + Q with large complete active space self-consistent field (CASSCF) active space provides results in excellent agreement with the full CI method
Before we present the DFT POD results for HAB79, we investigate the performance of different XC functionals and basis sets on two smaller databases of pi-conjugated organic homo-dimers for which similar ab initio reference data are available, HAB11 and HAB7
Summary
One of the key parameters in molecular charge transport modeling and simulation is the electronic coupling between two diabatic charge transfer electronic states. Electronic couplings (or charge transfer integrals) for a given molecular dimer configuration can be calculated using a variety of methods, including but not limited to ab initio wavefunction, density functional theory (DFT), and semi-empirical or solely empirical methods. The choice of a particular method is typically dictated by various factors, trying to strike a balance between chemical accuracy and computational resource utilization. Electronic couplings (or charge transfer integrals) for a given molecular dimer configuration can be calculated using a variety of methods, including but not limited to ab initio wavefunction, density functional theory (DFT), and semi-empirical or solely empirical methods.. We introduce the HAB79 dataset: a selection of 79 heterocyclic polyaromatic hydrocarbons, either already utilized or inspired by contemporary organic (opto)electronic applications, arranged to 921 structurally diverse homo-molecular dimers. Having such structural information at hand, ab initio electronic couplings are calculated at the CASSCF/NEVPT2 level of theory, forming this way the HAB79 charge transfer integral reference dataset.
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