Accelerating Pythonic Coupled-Cluster Implementations: A Comparison Between CPUs and GPUs.

In this work, we benchmark several Python routines for time and memory requirements to identify the optimal choice of the tensor contraction operations available. We scrutinize how to accelerate the bottleneck tensor operations of Pythonic coupled-cluster implementations in the Cholesky linear algebra domain, utilizing a NVIDIA Tesla V100S PCIe 32GB (rev 1a) graphics processing unit (GPU). The NVIDIA compute unified device architecture API interacts with CuPy, an open-source library for Python, designed as a NumPy drop-in replacement for GPUs. Due to the limitations of video memory, the GPU calculations must be performed batch-wise. Timing results of some contractions containing large tensors are presented. The CuPy implementation leads to a factor of 10-16 speed-up of the bottleneck tensor contractions compared to computations on 36 central processing unit (CPU) cores. Finally, we compare example CCSD and pCCD-LCCSD calculations performed solely on CPUs to their CPU-GPU hybrid implementation, which leads to a speed-up of a factor of 3-4 compared to the CPU-only variant.

Propagation of Holes and Electrons in Metal-Organic Frameworks.

Charge transport in two zinc metal-organic frameworks (MOFs) has been investigated using periodic semiempirical molecular orbital calculations with the AM1* Hamiltonian. Restricted Hartree-Fock calculations underestimate the band gap using Koopmans theorem (ca. 2 eV compared to the experimental value of 2.8 eV). However, it almost doubles when the constraint on the wave function to remain spin-restricted is removed and the energies of the UHF Natural Orbitals are used. Charge-transport simulations using propagation of the electron- or hole-density in imaginary time allow charge-transport paths and mechanisms to be determined. The calculated relative mobilities in the directions of the three crystal axes agree with experimental expectations, but the absolute values are not reliable using the current technique. Hole-mobility along the crystal c-axis (along the metal stacks) is found to be 13 times higher in the zinc MOF with anthracene linker (Zn-ANMOF-74) than in the other directions, whereas the factor is far smaller (1.7) for electron mobility. Directional preferences are far less distinct in the equivalent structure with phenyl linkers (Zn-MOF-74). The imaginary-time simulation technique does not give quantitative mobilities. The simulations reveal a change in mechanism between the different directions: Coherent polaron migration is observed along the stacks but tunneling hops between them.

Correction to: The Feynman dispersion correction for MNDO extended to F, Cl, Br and I.

A small coding error in the development version of EMPIRE led to some inconsistencies in the above article. They are corrected in this erratum.

The Feynman dispersion correction for MNDO extended to F, Cl, Br and I.

The recently introduced "Feynman" dispersion correction for MNDO (MNDO-F) has been extended to include the elements fluorine, chlorine, bromine and iodine and the original parameterization for hydrogen, carbon, nitrogen and oxygen improved by allowing individual damping radii for the elements. MNDO-F gives a root-mean-square deviation to reference interaction energies of 0.35 kcal mol-1 for the complete parameterization dataset of H, C, N, O, F, Cl, Br and I containing compounds. Graphical Abstract The electrostatic potential at the 0.001 a.u. isodensity surface of the π-complex between benzene and 1,3,5-triodobenzene calculated at the MNDO-F optimized geometry.

A Feynman dispersion correction: a proof of principle for MNDO.

A dispersion correction is introduced and tested for MNDO. The shift in electron density caused by the interaction between oscillating dipoles in the London picture of dispersion is mimicked by adding a small r-7-dependent attractive nucleus-electron potential to the core Hamiltonian. This potential results in a shift in electron density similar to that used by Feynman to explain dispersion. The resulting parameterized self-consistent and inherently multicenter treatment (MNDO-F) gives good results for CHNO compounds that do not exhibit hydrogen bonds, which MNDO cannot reproduce. This "Feynman" dispersion correction is also applicable to Hartree-Fock and density functional theory. Graphical abstract The MNDO-F optimized geometry for a C60-fullerene tetramer in a tetrahedral configuration.

Charge Transport in Organic Materials: Norm-Conserving Imaginary Time Propagation with Local Ionization Energy as the External Potential.

An additional charge carrier described as its wave function is propagated in imaginary time using stepwise matrix multiplication and a correction to ensure that the simulation is norm-conserving. The propagation Hamilton operator uses the local ionization energy of a rubrene single crystal, calculated with semiempirical molecular orbital theory, as an external potential for holes to model the interaction with the underlying molecular structure. Virtual electrodes are modeled by setting the potentials in the appropriate areas to constant values with the difference corresponding to the source-drain voltage. Although imaginary time cannot be interpreted directly as time, the simulated gate-dependent imaginary transfer rate is in acceptable qualitative agreement with the experimentally measured gate-dependent hole-transfer rate through a rubrene single crystal.