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Register a new userBenchmark tests of a strongly constrained semilocal functional with a long-range dispersion correction
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Added by
Jan Gerit Brandenburg
Publication date
2016
fields
Physics
and research's language is
English
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The strongly constrained and appropriately normed (SCAN) semilocal density functional [J. Sun, A. Ruzsinszky, J. P. Perdew textit{Phys. Rev. Lett.} {bf 115}, 036402 (2015)] obeys all 17 known exact constraints for meta-generalized-gradient approximations (meta-GGA) and includes some medium range correlation effects. Long-range London dispersion interactions are still missing, but can be accounted for via an appropriate correction scheme. In this study, we combine SCAN with an efficient London dispersion correction and show that lattice energies of simple organic crystals can be improved with the applied correction by 50%. The London-dispersion corrected SCAN meta-GGA outperforms all other tested London-dispersion corrected meta-GGAs for molecular geometries. Our new method delivers mean absolute deviations (MADs) for main group bond lengths that are consistently below 1,pm, rotational constants with MADs of 0.2%, and noncovalent distances with MADs below 1%. For a large database of general main group thermochemistry and kinetics, it also delivers a weighted mean absolute deviation below 4 kcal/mol, one of the lowest for long-range corrected meta-GGA functionals. We also discuss some consequences of numerical sensitivity encountered for meta-GGAs.
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The Strongly Constrained and Appropriately Normed (SCAN) functional is a non-empirical meta-generalized-gradient approximation (meta-GGA) functional that satisfies all the known constraints that a meta-GGA functional can, but it also exhibits a great degree of sensitivity to numerical grids. Its numerical complexities are amplified when used in Perdew-Zunger (PZ) self-interaction correction (SIC) which requires evaluating energies and potentials using orbital densities that vary far more rapidly than spin densities. Recent regularization of the SCAN functional (rSCAN) simplifies numerical complexities of SCAN at the expense of violation of some exact constraints. To develop a good understanding of the performance of rSCAN and the effect of loss of an exact constraint at the limit of slowly varying density, we have compared its performance against SCAN for vibrational frequencies, infra-red and Raman intensities of water clusters, electric dipole moments, spin magnetic moments of a few molecular magnets, weak interaction energies of dimers, barrier heights of reactions, and atomization energies for benchmark sets of molecules. Likewise, we examined the performance of SIC-rSCAN using the PZ-SIC method by studying atomic total energies, ionization potentials and electron affinities, molecular atomization energies, barrier heights, and dissociation and reaction energies. We find that rSCAN requires a much less dense numerical grid and gives very similar results as SCAN for all properties examined with the exception of atomization energies which are somewhat worse in rSCAN. On the other hand, SIC-rSCAN gives marginally better performance than SIC-SCAN for almost all properties studied in this work.
In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This effect results in distinctive signatures in the vibrational modes of the polymer. We probe polaron photo- generation dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 200 fs, which is nearly two orders of magnitude faster than exciton localisation in the neat polymer film. Surprisingly, further vibrational evolution on <50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not signif- icantly different from that in equilibrium. This suggests that charges are free from their mutual Coulomb potential, under which vibrational dynamics would report charge-pair relaxation. Our work addresses current debates on the photocarrier generation mechanism at organic semiconductor heterojunctions, and is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.
We find that the recently developed self consistent and appropriately normed (SCAN) meta-generalized gradient approximation, which has been found to provide highly accurate results for many materials, is, however, not able to describe the stability and properties of phases of Fe important for steel. This is due to an overestimated tendency towards magnetism and exaggeration of magnetic energies, which we also find in other transition metals.
Self-interaction (SI) error, which results when exchange-correlation contributions to the total energy are approximated, limits the reliability of many density functional approximations. The Perdew-Zunger SI correction (PZSIC), when applied in conjunction with the local spin density approximation (LSDA), improves the description of many properties, but overall, this improvement is limited. Here we propose a modification to PZSIC that uses an iso-orbital indicator to identify regions where local SI corrections should be applied. Using this local-scaling SIC (LSIC) approach with LSDA, we analyze predictions for a wide range of properties including, for atoms, total energies, ionization potentials, and electron affinities, and for molecules, atomization energies, dissociation energy curves, reaction energies, and reaction barrier heights. LSIC preserves the results of PZSIC-LSDA for properties where it is successful and provides dramatic improvements for many of the other properties studied. Atomization energies calculated using LSIC are better than those of the Perdew, Burke, and Ernzerhof (PBE) generalized gradient approximation (GGA) and close to those obtained with the Strongly Constrained and Appropriately Normed (SCAN) meta-GGA. LSIC also restores the uniform gas limit for the exchange energy that is lost in PZSIC-LSDA. Further performance improvements may be obtained by an appropriate combination or modification of the local scaling factor and the particular density functional approximation.
We present PyCDFT, a Python package to compute diabatic states using constrained density functional theory (CDFT). PyCDFT provides an object-oriented, customizable implementation of CDFT, and allows for both single-point self-consistent-field calculations and geometry optimizations. PyCDFT is designed to interface with existing density functional theory (DFT) codes to perform CDFT calculations where constraint potentials are added to the Kohn-Sham Hamiltonian. Here we demonstrate the use of PyCDFT by performing calculations with a massively parallel first-principles molecular dynamics code, Qbox, and we benchmark its accuracy by computing the electronic coupling between diabatic states for a set of organic molecules. We show that PyCDFT yields results in agreement with existing implementations and is a robust and flexible package for performing CDFT calculations. The program is available at https://github.com/hema-ted/pycdft/.
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