Calculations Of Effective Exchange Integrals Research Articles

DFT calculations of effective exchange integrals at the complete basis set limit on oxo-vanadium ring complex

A new scheme for calculations of effective exchange integral (J) values at the complete basis set (CBS) limit is proposed based on an extrapolation method of the total energy and an approximate spin projection (AP) method. By using the scheme, the J values between oxo-vanadium units in a (VO)7 ring complex at the CBS limit are calculated with several hybrid density functional theory (DFT) functional sets that are BHandHLYP, B3LYP and CAM-B3LYP. The calculated results indicate that the J values at the CBS limit with BHandHLYP reproduces the experimental result well. In addition, some basis sets such as Dunning’s correlation-consistent polarized valence, Pople series and def2 series are also examined to elucidate which basis function reproduces the J values at the CBS limit. The results indicate that the basis sets that are larger than 6-31+G∗ level are at least required to reproduce the J values of the (VO)7 complex at the CBS limit.

BS DFT and BS HDFT studies of CrCr sextuple bond from the viewpoint of electron correlation effects

AbstractThe nature of CrCr bond and effectiveness of broken‐symmetry (BS) density functional theory (DFT) and hybrid DFT (HDFT) methods are examined in terms of electron correlation effects. Two types of correlation energies, i.e., static and dynamical correlation energies are estimated by relative energy curves and correlation energy curves. Energy decomposition analyses and natural orbital analyses reveal how these electron correlation effects affect the CrCr bonding. In addition, the effect of those effects to an energy gap between the lowest spin (LS) state (S = 0) and the highest spin (HS) state (S = 6) is also discussed with respect to calculations of effective exchange integrals (J). © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods II: Cluster models of Jahn–Teller distorted K 2CuF 4 solid

In the strong correlated perovskite-type copper fluoride of K 2CuF 4, the degree of freedom in d orbitals of copper cation should be considered to understand the magnetism. We performed the cluster model calculations under consideration of octahedral fluorine anions around copper cation, in order to investigate the magnetism. At the amplitude of Jahn–Teller distortion, the alternate d z 2 - x 2 type orbital ordering and the experimental effective exchange integral ( J ab ) value were reproduced. The calculated potential energy curves were discussed, from the viewpoint of the theoretical total energy in the Jahn–Teller distorted structure proposed by Kanamori.

Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods. III. Cluster models of three-dimensional KNiF3 solid.

Previously, we have performed the spin-polarized hybrid-density functional theory (HUDFT) calculations for elucidating magnetic properties of the two-dimensional (2D) K2NiF4 and K2CuF4 solids. In Part I, it has been concluded that the half-and-half-(HH-) type HUDFT method is one of the best calculation methods for these species. On the other hand, in Part II, we have demonstrated that potential curves for cluster models of K2CuF4 and KCuF3 are reasonably calculated by the HH-type HUDFT method under the approximate spin projection, and the lattice distortion resulted by Jahn-Teller effect is expressed as the second-order polynomial. In this study, we pay attention to the three-dimensional (3D) magnetic interactions in KNiF3. Our effective exchange integral Jab schemes for 3D cluster models such as KNi8F12 (4) with and without point charges have provided the reasonable Jab values (-30.24-34.48 cm(-1)), in comparison with the experimental one (-30.58 cm(-1)). The 3D magnetic interactions have been investigated from viewpoints of the Mulliken spin density and charge density populations, the natural orbital analysis, and chemical indices. Point charges located in positions of fluorine anion adjacent to cluster model have reproduced Jab values well. Roles of potassium as counter cation in KNiF3 solid were also investigated. It was concluded that potassium has a role of stabilizing the 3D magnetic structures. Finally, the mutual relationships between broken-symmetry and symmetry-adapted approaches are discussed on the basis of chemical indices.

Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods. I. Cluster models of K2NiF4-type solids

Previously, various symmetry-adapted (SA) and broken-symmetry (BS) computations have been performed for strongly correlated transition metal species so as to examine magnetic properties in simple cluster models such as binuclear transition metal oxides. Though SA computations such as the complete active space configuration interaction and CASSCF are desirable for estimating physical constants, these computations are heavy for larger cluster models with strongly correlated electrons. K2NiF4 is known to be the two-dimensional perovskite-type antiferromagnet and to have the electronic configuration similar to that of La2CuO4. Here, we have examined the utility and applicability of the BS spin-polarized hybrid-density functional theory (HUDFT) for cluster models of K2NiF4. As the result, HUDFT calculation such as UB2LYP has provided the reasonable effective exchange integral (Jab) followed by our approximately spin projected scheme, in comparison to the experimental one. It was also found that the square planar tetranuclear model has provided the most reasonable Jab value by HUDFT. In addition, external effects such as putting point charges around cluster models and changing distances between nickel and fluorine have been also examined. The natural orbital analysis by HUDFT has been carried out to obtain natural orbitals and their occupation numbers. Charge density, spin density, and chemical indices expressed by the occupation numbers have been also obtained to elucidate the nature of the chemical bonds in the K2NiF4-type solids.

Theoretical Calculation of Effective Exchange Integrals for One-and Two-Dimensional Poly(Phenylenemethylene) Systems. Possibilities of Organic Ferro-and Ferri-Magnetic Solids

Abstract Ab initio post UHF, DFT, CASSCF and semiempirical INDO calculations were carried out for one-and two-dimensional poly(phenylenmethylene) in order to elucidate variations of the effective exchange integrals with molecular connectivity, cluster size and hole (electron) doping. It was shown that the ferromagnetic effective exchange integrals by UCCSD(T) and DFT/4–31G are about 400 cm−1 for m-phenylene bis(methylene). The nigh-spin ground states of hole-doped polycarbenes were also studied, indicating a possibility of obtaining the high Tc organic ferrimagnets induced by the one electron transfer between polycarbene and spacer (electron donor or acceptor).

Comparison between spin restricted and unrestricted post-Hartree—Fock calculations of effective exchange integrals in Ising and Heisenberg models

Spin-restricted and unrestricted post-Hartree—Fock calculations were carried out for clusters of triplet methylene and nitroxide radicals. The UHF-based methods such as UMP and QCISD followed by approximate spin projection provide reasonable energy differences between the high-and low-spin states of these species. They are close to the corresponding values from spin-restricted multi-reference (MR) approaches such as CASSCF and second-order (SO) CI. Implications of SOCI and MRSDCI results are discussed in relation to the size inconsistency erros involved in ab initio calculations of weak interaction energies, such as the effective exchange integrals in Ising and Heisenberg models.

Theoretical calculations of effective exchange integrals between nitronyl nitroxides with donor and acceptor groups

Semi-empirical calculations on the basis of an extended McConnell model have been carried out for dimers of phenyl nitronyl nitroxides with donor or acceptor groups. The intermolecular effective exchange integrals are positive (ferromagnetic) for appropriately stacked forms of these species because of large spin polarization effects. Implications of these results are discussed in relation to the organic ferromagnetism discovered recently for para-nitrophenyl nitronyl nitroxide.

Ab initio calculations of effective exchange integrals for triplet carbene clusters. Importance of stacking modes for ferromagnetic interactions

An ab initio method is presented to calculate the effective exchange integrals ( J ab ) for exchange-coupled clusters on the basis of the approximately projected UHF (APUHF) and UHF Møller-Plesset (APUMP) procedures. The APUMP method is applied to the calculation of the effective exchange integrals for clusters of triplet carbenes in order to elucidate variations of J ab values with the basis set and correlation corrections. The signs and magnitudes of the J ab values calculated for clusters of phenyl carbenes by the APUHF STO-3G method are found to be compatible with the observed data, indicating the importance of stacking modes for ferromagnetic interactions.

Ab initio molecular orbital calculations of effective exchange integrals for transition metal oxides and halides: Strong superexchange interactions and high T c superconductivity

Ab initio unrestricted Hartree-Fock (UHF) calculations involving the correlation corrections by the Møller-Plesset (MP) perturbation method are carried out for K 2NiF 4-type transition-metal halides and related copper oxides. The magnitude of effective exchange integrals (Jab) obtained by the spin-projected UHF MP method is found to be particularly large for copper oxides, in agreement with recent experiments. Implications of these results are discussed in relation to spin-mediated theories for the mechanism of high-T c superconductivity of copper oxides.

Ab initio molecular orbital calculations of effective exchange integrals between transition metal ions

A theory is presented to calculate the effective exchange integrals ( J ab) for exchange-coupled metal clusters on the basis of the spin-projected unrestricted Hartree-Fock (PUHF) molecular orbital method. It is applied to the calculation of the effective exchange integrals for binuclear transition metal complexes such as μ-oxo dimers. The signs and magnitudes of the J ab values thus calculated are found to be compatible with the observed data.