Why the pyrochlore-like antiferromagnet NaCu3F7is magnetically non-frustrated.

We present a theoretical study of the magnetic properties for the pyrochlore-like NaCu3F7compound, which surprisingly experience little or no frustration. The magnetic effective exchange interactions were calculated usingab-initiomethods explicitly treating the electronic correlation. A model Hamiltonian was built from these interactions and used to determine the zero temperature magnetic order versus magnetic field, using a quantum Heisenberg Hamiltonian or, for comparison, a spin 1/2 Ising Hamiltonian. The magnetic order at zero magnetic field is non frustrated and associated with the propagation vectorq= (0,0,0). The magnetization versus magnetic field reveals the existence of a 1/3 plateau that could be observed in high-pulsed magnetic field experiments. Analysing the magnetic interactions, we highlight the importance of the magnetic-ion nature and the lattice distortion, in the non-frustrated nature of the NaCu3F7magnetic structure, despite its triangular/Kagome subnetworks. We believe that this non-frustrated behaviour could also take place in other triangular copper-based systems. .

For an ab initio calculation of the magnetic excitations: RelaxSE!

In this paper, we present a novel efficient and parallel implementation, RelaxSE, for the calculation of the low-lying excited states and energies of strongly correlated systems. RelaxSE is based on the fully uncontracted multi-reference method of Selected Active Space + Single excitations. This method has been specifically designed to be able to tackle systems with numerous open shells per atoms. It is, however, computationally challenging due to the rapid scaling of the number of determinants and their non-trivial ordering induced by the selection process. We propose a combined determinant-driven and integral-driven approach designed for hybrid OpenMP/MPI parallelization. The performances of RelaxSE are evaluated on a controlled test set and show linear scaling with respect to the number of determinants and a small overhead due to the parallelization. Systems with up to 1 × 109 determinants are successfully computed.

Pressure-dependent X-ray diffraction of the multiferroics RMn2O5.

The room-temperature structural properties of the RMn2O5 multiferroics have been investigated under pressure, using powder X-ray scattering and density functional theory (DFT) calculations. It was possible to determine the lattice parameters and the main atomic positions as a function of pressure. Good agreement was observed between the X-ray and DFT results for most of the determined crystallographic data. From the DFT calculations, it was possible to infer the pressure evolution of the exchange interactions, and this analysis led to the conclusion that the onset of the q = (½, 0, ½) magnetic structure under pressure is related to the increase in the J1 super-exchange terms (due to the reduction in the Mn-O distances) compared with the Mn-R exchange interactions. In addition, the 1D antiferromagnetic character of the compounds should be reinforced under pressure.

Divide-Expand-Consolidate Second-Order Møller-Plesset Theory with Periodic Boundary Conditions.

We present a generalization of the divide-expand-consolidate (DEC) framework for local coupled-cluster calculations to periodic systems and test it at the second-order Møller-Plesset (MP2) level of theory. For simple model systems with periodicity in one, two, and three dimensions, comparisons with extrapolated molecular calculations and the local MP2 implementation in the Cryscor program show that the correlation energy errors of the extended DEC (X-DEC) algorithm can be controlled through a single parameter, the fragment optimization threshold. Two computational bottlenecks are identified: the size of the virtual orbital spaces and the number of pair fragments required to achieve a given accuracy of the correlation energy. For the latter, we propose an affordable algorithm based on cubic splines interpolation of a limited number of pair-fragment interaction energies to determine a pair cutoff distance in accordance with the specified fragment optimization threshold.

Comparison of Three Efficient Approximate Exact-Exchange Algorithms: The Chain-of-Spheres Algorithm, Pair-Atomic Resolution-of-the-Identity Method, and Auxiliary Density Matrix Method.

We compare the performance of three approximate methods for speeding up evaluation of the exchange contribution in Hartree-Fock and hybrid Kohn-Sham calculations: the chain-of-spheres algorithm (COSX; Neese , F. Chem. Phys. 2008 , 356 , 98 - 109 ), the pair-atomic resolution-of-identity method (PARI-K; Merlot , P. J. Comput. Chem. 2013 , 34 , 1486 - 1496 ), and the auxiliary density matrix method (ADMM; Guidon , M. J. Chem. Theory Comput. 2010 , 6 , 2348 - 2364 ). Both the efficiency relative to that of a conventional linear-scaling algorithm and the accuracy of total, atomization, and orbital energies are compared for a subset containing 25 of the 200 molecules in the Rx200 set using double-, triple-, and quadruple-ζ basis sets. The accuracy of relative energies is further compared for small alkane conformers (ACONF test set) and Diels-Alder reactions (DARC test set). Overall, we find that the COSX method provides good accuracy for orbital energies as well as total and relative energies, and the method delivers a satisfactory speedup. The PARI-K and in particular ADMM algorithms require further development and optimization to fully exploit their indisputable potential.

Range-separated time-dependent density-functional theory with a frequency-dependent second-order Bethe-Salpeter correlation kernel.

We present a range-separated linear-response time-dependent density-functional theory (TDDFT) which combines a density-functional approximation for the short-range response kernel and a frequency-dependent second-order Bethe-Salpeter approximation for the long-range response kernel. This approach goes beyond the adiabatic approximation usually used in linear-response TDDFT and aims at improving the accuracy of calculations of electronic excitation energies of molecular systems. A detailed derivation of the frequency-dependent second-order Bethe-Salpeter correlation kernel is given using many-body Green-function theory. Preliminary tests of this range-separated TDDFT method are presented for the calculation of excitation energies of the He and Be atoms and small molecules (H2, N2, CO2, H2CO, and C2H4). The results suggest that the addition of the long-range second-order Bethe-Salpeter correlation kernel overall slightly improves the excitation energies.

Excitation energies along a range-separated adiabatic connection.

We present a study of the variation of total energies and excitation energies along a range-separated adiabatic connection. This connection links the non-interacting Kohn-Sham electronic system to the physical interacting system by progressively switching on the electron-electron interactions whilst simultaneously adjusting a one-electron effective potential so as to keep the ground-state density constant. The interactions are introduced in a range-dependent manner, first introducing predominantly long-range, and then all-range, interactions as the physical system is approached, as opposed to the conventional adiabatic connection where the interactions are introduced by globally scaling the standard Coulomb interaction. Reference data are reported for the He and Be atoms and the H2 molecule, obtained by calculating the short-range effective potential at the full configuration-interaction level using Lieb's Legendre-transform approach. As the strength of the electron-electron interactions increases, the excitation energies, calculated for the partially interacting systems along the adiabatic connection, offer increasingly accurate approximations to the exact excitation energies. Importantly, the excitation energies calculated at an intermediate point of the adiabatic connection are much better approximations to the exact excitation energies than are the corresponding Kohn-Sham excitation energies. This is particularly evident in situations involving strong static correlation effects and states with multiple excitation character, such as the dissociating H2 molecule. These results highlight the utility of long-range interacting reference systems as a starting point for the calculation of excitation energies and are of interest for developing and analyzing practical approximate range-separated density-functional methodologies.

Assessment of range-separated time-dependent density-functional theory for calculating C6 dispersion coefficients.

We assess a variant of linear-response range-separated time-dependent density-functional theory (TDDFT), combining a long-range Hartree-Fock (HF) exchange kernel with a short-range adiabatic exchange-correlation kernel in the local-density approximation (LDA) for calculating isotropic C6 dispersion coefficients of homodimers of a number of closed-shell atoms and small molecules. This range-separated TDDFT tends to give underestimated C6 coefficients of small molecules with a mean absolute percentage error of about 5%, a slight improvement over standard TDDFT in the adiabatic LDA which tends to overestimate them with a mean absolute percentage error of 8%, but close to time-dependent Hartree-Fock which has a mean absolute percentage error of about 6%. These results thus show that introduction of long-range HF exchange in TDDFT has a small but beneficial impact on the values of C6 coefficients. It also confirms that the present variant of range-separated TDDFT is a reasonably accurate method even using only a LDA-type density functional and without adding an explicit treatment of long-range correlation.

Solid-phase synthesis of peptide-viologen conjugates.

This paper presents a robust method for the conjugation of viologens to peptides using an amide coupling strategy that is compatible with standard Fmoc solid-phase peptide synthesis. Methodology is presented for monitoring the milligram scale process quantitatively by UV spectroscopy. This chemistry enables the synthesis of a broad range of asymmetric viologens in high yield at room temperature and is compatible with a wide range of functional groups, including amine, guanidinyl, thiol, carboxylic acid, phenol, and indole.