Dage Sundholm

Calculation of spin-current densities using gauge-including atomic orbitals

A method for calculating the various components of the magnetically induced current-density tensor using gauge-including atomic orbitals is described. The method is formulated in the framework of analytical derivative theory, thus enabling implementation at the Hartree–Fock self-consistent-field (HF-SCF) as well as at electron-correlated levels. First-order induced current densities have been computed up to the coupled-cluster singles and doubles level (CCSD) augmented by a perturbative treatment of triple excitations [CCSD(T)] for carbon dioxide and benzene and up to the full coupled-cluster singles, doubles, and triples (CCSDT) level in the case of ozone. The applicability of the gauge including magnetically induced current method to larger molecules is demonstrated by computing first-order current densities for porphin and hexabenzocoronene at the HF-SCF and density-functional theory level. Furthermore, a scheme for obtaining quantitative values for the induced currents in a molecule via numerical inte...

Density functional theory calculations of the visible spectrum of chlorophyll a

Abstract The molecular structure of chlorophyll a including the phytyl chain has been fully optimized at density-functional level. The excitation energies and oscillator strengths obtained at density-functional time-dependent perturbation theory level are in good agreement with experiment. The present calculations show that Goutermans generally accepted four-orbital model does not correctly describe the excitation spectrum of chlorophyll a . Thirteen excited states are found in the visible region: four are found in the long-wavelength region, two weak transitions are obtained at about 490 nm, while seven transitions are in the Soret band region.

ROVIBRATIONALLY AVERAGED NUCLEAR MAGNETIC SHIELDING TENSORS CALCULATED AT THE COUPLED-CLUSTER LEVEL

Nuclear magnetic shielding tensor functions for H2, HF, N2, CO, and F2 are calculated at the coupled‐cluster singles and doubles level augmented by a perturbative correction for triple excitations [CCSD(T)]. The shielding constants for the lowest rovibrational states of these diatomics are obtained by solving the rovibrational Schrodinger equation with the finite‐element method. For H, C, and F, absolute scales for the nuclear magnetic shielding constants have been obtained by combining computed diamagnetic shieldings with paramagnetic contributions deduced from measured spin‐rotation constants and calculated rovibrational corrections. Since the experimental spin‐rotation constants for N2 and CO are inaccurate, shielding scales for N and O based on coupled cluster calculations are probably the most accurate available.

The gauge including magnetically induced current method

An overview of applications of the recently developed gauge including magnetically induced current method (GIMIC) is presented. The GIMIC method is used to obtain magnetically induced current densities in molecules. It provides detailed information about electron delocalization, aromatic character, and current pathways in molecules. The method has been employed in aromaticity studies on hydrocarbons, complex multi-ring organic nanorings, Möbius twisted molecules, inorganic and all-metal molecular rings and open-shell species. Recent studies on hydrogen-bonded molecules indicate that GIMIC can also be used to estimate hydrogen-bond strengths without fragmentation of the system. Preliminary results are presented on the applicability of GIMIC for investigating current transport in molecules attached to clusters simulating molecular conductivity measurements. Advantages and limitations of the GIMIC method are reviewed and discussed.

Magnetically Induced Current Densities in Aromatic, Antiaromatic, Homoaromatic, and Nonaromatic Hydrocarbons

The magnetically induced current densities for ring-shaped hydrocarbons are studied at the density functional theory (DFT) and second-order Møller-Plesset (MP2) levels using gauge-including atomic orbitals. The current densities are calculated using the gauge-including magnetically induced current approach. The calculations show that all studied hydrocarbon rings sustain strong diatropic and paratropic ring currents when exposed to an external magnetic field, regardless whether they are unsaturated or not. For nonaromatic rings, the strength of the paratropic current flowing inside the ring is as large as the diatropic one circling outside it, yielding a vanishing net ring current. For aromatic molecules, the diatropic current on the outside of the ring is much stronger than the paratropic one inside, giving rise to the net diatropic ring current that is typical for aromatic molecules. For antiaromatic molecules, the paratropic ring-current contribution inside the ring dominates. For homoaromatic molecules, the diatropic current circles at the periphery of the ring. The ring current is split at the CH(2) moiety; the main fraction of the current flow passes outside the CH(2) at the hydrogens, and some current flows inside the carbon atom. The diatropic current does not take the through-space short-cut pathway, whereas the paratropic current does take that route. Calculations of the ring-current profile show that the ring current of benzene is not transported by the pi electrons on both sides of the molecular ring. The strongest diatropic ring current flows on the outside of the ring and in the ring plane. A weaker paratropic current circles inside the ring with the largest current density in the ring plane. Due to the ring strain, small unconjugated and saturated hydrocarbon rings sustain a strong ring current which could be called ring-strain current. Nuclear magnetic shieldings calculated for 1,3,5-cycloheptatriene and homotropylium at the DFT and MP2 levels agree well with experimental values.

Ab initio determination of the induced ring current in aromatic molecules

The aromatic ring current shieldings (ARCS) approach is a new method to determine the strength of the induced ring current which is related to the molecular aromaticity. In the ARCS method, the strength of the induced aromatic ring current and the size of the current ring are obtained from nuclear magnetic shieldings calculated in discrete points along a line perpendicular to the molecular plane starting at the center of the molecule. The induced ring currents are used to determine the relative degree of aromaticity for benzene, furan, pyrrole, cyclohexane, cyclohexadiene, thiophen, p-benzoquinone, cyclopentadienide anion, cycloheptatrienyl cation, 1,4-dihydronaphthalene, and prophin.

Two-dimensional, fully numerical molecular calculations. X: Hartree-Fock results for He2, Li2, Be2, HF, OH−, N2, CO, BF, NO+ and CN−

Fully numerical two-dimensional Hartree-Fock calculations are reported for the ground states of the diatomic systems He2, Li2, Be2, HF, OH-, N2, CO, BF, NO+ and CN-. total energies, electric multipole moments and electric field gradients are given. A close agreement with the seminumerical results by McCullough is obtained. High multipole moments, up to Q 10, are reported and compared to LCAO ones. The role of correlation in nuclear quadrupole coupling constants is discussed.

REPULSIVE INTERATOMIC POTENTIALS CALCULATED USING HARTREE-FOCK AND DENSITY-FUNCTIONAL THEORY METHODS

The repulsive part of the interatomic potential affects the outcome of computer simulations of many irradiation processes of practical interest, like sputtering and ion irradiation range distributions. The accuracy of repulsive potentials is studied by comparing potentials calculated using commonly available density-functional theory (DFT) and Hartreee Fock (HF) methods to highly accurate fully numerical HF and Hartree-Fock-Slater (HFS) calculations. We find that DFT calculations utilizing numerical basis sets and HF calculations using deconrrncted standard basis sets provide repulsive potentials which are significantly improved compared to the standard universal ZBL potential. The accuracy of the calculated potentials is almost totally governed by the quality of the one-particle basis set. The use of reliable repulsive potentials open up new avenues for analysis of ion irradiation experiments. 0 1997 Elsevier Science B.V.

A Numerical Hartree-Fock Program for Diatomic Molecules

This paper describes an algorithm and a computer program which solves numerically (virtually exactly) equations of the restricted open-shell Hartree-Fock and Hartree-Fock-Slater model for diatomic molecules

Properties of WAu12

The icosahedral cluster-compound WAu12 was recently predicted by Pyykko and Runeberg and experimentally prepared in the gas phase by the group of Lai-Sheng Wang. The photoelectron spectra and electron affinity were reported; the other physical properties remain unknown. Anticipating further experimental studies on it, we report here predicted vibrational spectra, NMR chemical shifts, spin–spin coupling constants and quadrupole coupling constants as well as optical spectra at the level of single and double excitations. The population analysis is non-trivial. By direct numerical integration, a charge of roughly +1 is obtained for the central tungsten atom. The charge distribution is strongly delocalised but bonding regions are clearly seen. A considerable electric field gradient exists at the gold nuclei. Although the radial bonds are strong, the system is quite elastic. The DFT activation energy for rotating one hemisphere against the other one, at a D5h transition state, is only about 20 kJ mol−1. The corresponding hu vibrational frequency is predicted to be slightly below 30 cm−1.