Exploring the Limits of DFT for Interaction Energies of Molecular Precursors to Organic Electronics

Neutral and charged assemblies of π-conjugated molecules span the field of organic electronics. Electronic structure computations can provide valuable information regarding the nature of the intermolecular interactions within molecular precursors to organic electronics. Here, we introduce a database of neutral (Pi29n) and radical (Orel26rad) dimer complexes that represent binding energies between organic functional units. The new benchmarks are used to test approximate electronic structure methods. Achieving accurate interaction energies for neutral complexes (Pi29n) is straightforward, so long as dispersion interactions are properly taken into account. However, π-dimer radical cations (Orel26rad) are examples of highly challenging situations for density functional approximations. The role of dispersion corrections is crucial, yet simultaneously long-range corrected exchange schemes are necessary to provide the proper dimer dissociation behavior. Nevertheless, long-range corrected functionals seriously underestimate the binding energy of Orel26rad at equilibrium geometries. In fact, only ωB97X-D, an empirical exchange-correlation functional fitted together with an empirical “classical” dispersion correction, leads to suitable results. Valuable alternatives are the more demanding MP2/6-31G*(0.25) level, as well as the most cost-effective combination involving a dispersion corrected long-range functional together with a smaller practical size basis set (e.g., LC-ωPBEB95-dDsC/6-31G*). The Orel26rad test set should serve as an ideal benchmark for assessing the performance of improved schemes.

Capturing Non-local Effects in Kohn-Sham Density Functional Theory

Andrey Laktionov

Density Functional Theory (DFT) and its time-dependent extension (TDDFT) have become two of the most popular approaches for computer simulations of the electronic structure and response properties of quantum systems. A reasonable compromise between accuracy and computational cost allows to apply DFT to a wide range of systems from small molecules to biological complexes. Despite the in principle exact nature of DFT and TDDFT, practical calculations require the use of approximate DFT exchange-correlation (XC) functionals and TDDFT kernels and the accuracy of the obtained results is determined by the accuracy of the chosen XC description. These approximations inevitably introduce some limitations in the use of DFT-based methods.The inability of the local spin density approximation, generalized gradient approximations (GGA) and even some hybrid functionals to properly describe charge transfer (CT) excitations and predict intermolecular interaction energies of weakly bound complexes are two major drawbacks of current XC descriptions. This thesis is therefore devoted to the improvement of XC functionals for the special cases of weak interactions and charge-transfer excitations. Many original strategies have been suggested to cure DFT calculations from these failures. In particular, the Dispersion-Corrected Atom-Centered Potentials (DCACP) approach provides an accurate description of dispersion forces within generalized gradient approximations for the exchange-correlation functional. The DCACP method has been extensively used for the last seven years and has shown an excellent performance for a large class of applications. However, one of the drawbacks of the current implementation of DCACP is that the correct R-6 asymptotics of dispersion interaction is not reproduced. A first goal of this thesis was to enable DCACP to recover the R-6 asymptotic limit. To this end, we have designed a new 2-channel version of DCACP and carried out test calculations on both small molecules and large macromolecular complexes. The obtained results demonstrate the excellent performance and transferability of the DCACP approach. Moreover, for large macromolecular complexes, in which the binding energy is dominated by dispersion, [pi]-stacking, or hydrogen bonding, 2-channels DCACP were found to be the best method overall for correcting the popular BLYP functional. Our study clearly shows that account of R-6 asymptotics is crucial for the description of large complexes and that 2-channels DCACP are fully able to capture these effects. On the contrary, an account of R-6 asymptotics is of little importance for small molecules since the remaining errors of the underlying GGA functional are dominating. With the aim of deepening our understanding of the DCACP concept and the reasons for its excellent performance, we explored the properties of the two DCACP parameters and were able to establish some systematic trends. It turned out that variational tuning of the DCACP can be done in an analytical manner that enables the easy generation of DCACP potentials for the full periodic table. Furthermore, since DCACP have little but crucial impact on the electronic density, dispersion energies can be obtained from non-self-consistent electron densities. These empirical findings suggest that the high transferability of DCACP is due to their atom-centered form and the intrinsically weak nature of dispersion interactions. [...]

EPFL2014

First-principles study of defects at the SiC/SiO2 interface through hybrid functionals

Fabien Devynck

The requirements of present high-performance power electronic systems are exceeding the power density, efficiency, and reliability of silicon-based devices. Silicon carbide (SiC) is a candidate of choice for high-temperature, high-speed, high-frequency, and high-power applications: it has a wide band gap, high thermal conductivity, high saturated electron drift velocity, and high breakdown electric field. SiC is also hard, chemically stable, and resistant to radiation damage. Despite these desirable electronic properties, SiC-based devices are still facing many performance challenges due to the high density of defect states at the SiC/SiO2 interface. This thesis is dedicated to the first-principles study of defects at the SiC/SiO2 interface through hybrid functionals. We first generate a model structure providing a realistic description of the majority bonding arrangements at the 4H(0001)-SiC/SiO2 interface. The description of the electronic structure of the model through hybrid functionals leads to band offsets in excellent agreement with their experimental counterparts. We then study several semiconductor and oxide defects using spin-polarized hybrid density functionals and identify candidate defects responsible for the high density of interface defects measured in the 4H-SiC band gap. Finally, the possibility of growing epitaxial silicate adlayers on SiC is investigated as an alternative approach to greatly reduce the high density of defect states at the SiC/SiO2 interface.

EPFL2008

Structural and electronic properties of an abrupt 4H-SiC(0001)/SiO2 interface model: Classical molecular dynamics simulations and density functional calculations

Peter Broqvist, Fabien Devynck, Alfredo Pasquarello

Using a density functional approach, we study structural and electronic properties of the 4H(0001)-SiC/SiO2 interface. Through the sequential use of classical and ab initio simulation methods, we generate an abrupt model structure which describes the transition between crystalline SiC and amorphous SiO2 without showing any coordination defect. The first step in our generation procedure consists in identifying suitable interfacial bonding patterns which account for the bond density reduction across the interface. In the second step, the connection to amorphous SiO2 is achieved through classical molecular dynamics. The atomic positions are then relaxed within a generalized gradient approximation of density functional theory. The final model structure shows good structural parameters and an oxide density typical of amorphous SiO2. We investigate the electronic structure of the generated model interface through the local density of states obtained with hybrid density functionals. In our atomically abrupt interface model, the full oxide band gap is recovered at a distance of similar to 5 A from the interface. This extent is in good agreement with estimates derived from internal photoemission measurements and provides support for the abrupt nature of the interface. We obtained band offsets through two different procedures: by evaluating the local density of states and by aligning the band extrema through the local electrostatic potential. Band offsets calculated with various functionals are compared to experimental values. The best agreement is achieved for a hybrid functional in which a screened Coulomb potential is used in the Hartree-Fock exchange term. For this case, the calculated band offsets underestimate the experimental values by similar to 25%, but agree with experiment within a few percent when expressed with respect to the oxide band gap.