Abstract: The Perdew-Zunger self-interaction correction to DFT energy functionals can improve the accuracy of calculated results in many respects. The long range effective potential for the electrons then has the correct -1/r dependence so Rydberg excited states of molecules and clusters of molecules can be accurately treated [1,2]. Also, localized electronic states are brought down in energy so defects in semi-conductors and insulators with defect states in the band gap can be characterized [3,4]. The calculations are, however, more challenging since the energy functional is no longer unitary invariant and each step in the self-consistent procedure needs to include an inner loop where unitary optimization is carried out [5,6]. As a result, the calculations produce a set of optimal orbitals which are generally localized and correspond well to chemical intuition. It has become evident that the optimal orbital need to be complex valued functions [7]. If they are restricted to real valued functions, the energy of atoms and molecules is less accurate and structure of molecules can even be incorrect [8]. [1] 'Self-interaction corrected density functional calculations of Rydberg states of molecular clusters: N,N-dimethylisopropylamine', H. Gudmundsdóttir, Y. Zhang, P. M. Weber and H. Jónsson, J. Chem. Phys. 141, 234308 (2014). [2] 'Self-interaction corrected density functional calculations of molecular Rydberg states', H. Gudmundsdóttir, Y. Zhang, P. M. Weber and H. Jónsson, J. Chem. Phys. 139, 194102 (2013). [3] `Simulation of Surface Processes', H. Jónsson, Proceedings of the National Academy of Sciences 108, 944 (2011). [4] 'Solar hydrogen production with semiconductor metal oxides: New directions in experiment and theory', Á. Valdés et al., Phys. Chem. Chem. Phys. 14, 49 (2012). [5] 'Variational, self-consistent implementation of the Perdew–Zunger self-interaction correction with complex optimal orbitals', S. Lehtola and H. Jónsson, Journal of Chemical Theory and Computation 10, 5324 (2014). [6] 'Unitary Optimization of Localized Molecular Orbitals', S. Lehtola and H. Jónsson, Journal of Chemical Theory and Computation 9, 5365 (2013). [7] 'Importance of complex orbitals in calculating the self-interaction corrected ground state of atoms', S. Klüpfel, P. J. Klüpfel and H. Jónsson, Phys. Rev. A Rapid Communication 84, 050501 (2011). [8] 'The effect of the Perdew-Zunger self-interaction correction to density functionals on the energetics of small molecules', S. Klüpfel, P. Klüpfel and H. Jónsson, J. Chem. Phys. 137, 124102 (2012).

Abstract: too high atomization energy (overbinding of the molecules), the application of PZ-SIC gives a large overcorrection and leads to significant underestimation of the atomization energy. The exchange enhancement factor that is optimal for the generalized gradient approximation within the Kohn-Sham (KS) approach may not be optimal for the self-interaction corrected functional. The PBEsol functional, where the exchange enhancement factor was optimized for solids, gives poor results for molecules in KS but turns out to work better than PBE in PZ-SIC calculations. The exchange enhancement is weaker in PBEsol and the functional is closer to the local density approximation. Furthermore, the drop in the exchange enhancement factor for increasing reduced gradient in the PW91 functional gives more accurate results than the plateaued enhancement in the PBE functional. A step towards an optimal exchange enhancement factor for a gradient dependent functional of the PZ-SIC form is taken by constructing an exchange enhancement factor that mimics PBEsol for small values of the reduced gradient, and PW91 for large values. The average atomization energy is then in closer agreement with the high-level quantum chemistry calculations, but the variance is still large, the F2 molecule being a notable outlier.

Abstract: Metal nanoparticles of only ~100-200 atoms are synthesized using a dendrimer encapsulation technique to facilitate a direct comparison with density functional theory (DFT) calculations in terms of both structure and catalytic function. Structural characterization is done using electron microscopy, x-ray scattering, and electrochemical methods. Combining these tools with DFT calculations is found to improve the quality of the structural models. DFT is also successfully used to predict trends between structure and composition of the nanoparticles and their catalytic function for reactions including the reduction of oxygen and the oxidation of formic acid. This investigation demonstrates some remarkable properties of the nanoparticles, including facile structural rearrangements and nanoscale tuning parameters which can be used to optimize catalytic rates.

Abstract: Despite many decades of force field developments, and the proliferation of efficient first principles molecular dynamics simulation techniques, a universal microscopic model for water in its various phases has not yet been achieved. In recent years, progress in force field development has shifted from optimizing in ever greater detail the parameters of simple pair-wise additive empirical potentials to developing more advanced models that explicitly include many-body interactions through induced polarization and short-range exchange-repulsion interactions. Such models are often parametrized to reproduce as closely as possible the Born-Oppenheimer surface from highly accurate quantum chemistry calculations; the best models often outperform DFT in accuracy, yet are orders of magnitude more computationally efficient. The SCME model was recently suggested as a physically rigorous and transparent model where the dominant electrostatic interaction is described through a single-center multipole expansion up to the hexadecapole moment, and where many-body effects are treated by induced dipole and quadrupole moments. In this paper, recent improvements of SCME are presented along with selected applications. Monomer flexibility is included via an accurate potential energy surface, a dipole moment surface is used to describe the geometric component of the dipole polarizability, and several formulations of the anisotropic short-range exchange-repulsion interaction are compared. The performance of this second version of the model, SCME2, is demonstrated by comparing to experimental results and high-level quantum chemistry calculations. Future perspectives for applications and developments of SCME2 are presented, including an outline for how the model can be adapted to describe mixed systems of water with other small molecules and how it can be used as a polarizable solvent in QM/MM simulations.

Abstract: The present study aims to explore the quantum topological features of the electron density and its Laplacian of the understudied molecular bromine species involved in ozone depletion events. The characteristics of the C-Br and O-Br bonds have been analyzed via quantum theory of atom in molecules (QTAIM) analysis using the wave functions computed at the B3LYP/aug-cc-PVTZ level of theory. Quantum topology analysis reveal that the C-Br and O-Br bonds show depletion of charge density indicating the increased ionic character of these bonds. Contour plots and relief maps have been analyzed for regions of valence shell charge concentrations (VSCC) and depletions (VSCD) in the ground state