Andrea Ligabue

The Dalton quantum chemistry program system

Dalton is a powerful general‐purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self‐consistent‐field, Møller–Plesset, configuration‐interaction, and coupled‐cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic‐structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge‐origin‐invariant manner. Frequency‐dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one‐, two‐, and three‐photon processes. Environmental effects may be included using various dielectric‐medium and quantum‐mechanics/molecular‐mechanics models. Large molecules may be studied using linear‐scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

Why Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators

Traces of magnetizability, traces of magnetic shielding at the hydrogen nuclei, and nucleus-independent chemical shift are not reliable aromaticity quantifiers for planar conjugated hydrocarbons. A measure of aromaticity is provided by the out-of-plane tensor components, whose magnitude is influenced by the pi-ring currents. The failure of nucleus-independent chemical shift in this regard was proved for the molecule shown in the abstract graphic, sustaining a diatropic pi-current. The validity of the ring-current model is reaffirmed. [structure: see text]

Correlated and gauge invariant calculations of nuclear magnetic shielding constants using the continuous transformation of the origin of the current density approach

We report for the first time an extended series of correlated origin independent calculations of nuclear magnetic shieldingtensors using the approach of continuous transformation of the origin of the current density to annihilate its diamagnetic contribution. A systematic study was undertaken to develop optimal basis sets for H, C, N, O, F correlated nuclear magnetic shieldings, looking for the best compromise between accuracy and size. Beyond Hartree–Fock calculations were carried out at two levels of accuracy, the multiconfigurational self-consistent field based on the complete active space scheme, and the second-order polarization propagator approximation (SOPPA). In addition we present the first shielding calculations employing the second-order polarization propagator approximation with coupled cluster singles and doubles amplitudes SOPPA (CCSD).

On the resolution of the optical rotatory power of chiral molecules into atomic terms. A study of hydrogen peroxide

An additive scheme for resolving average optical rotatory power of a molecule into atomic contributions, based on the acceleration gauge for the electric dipole, and/or the torque formalism, has been applied to hydrogen peroxide. Extended calculations have been carried out to test the reliability of the partition method. Gross atomic isotropic contributions to the average molecular property from oxygen and hydrogen atoms have been evaluated. The force and torque gauges provide different numerical values for atomic contributions.

Magnetic response of dithiin molecules: is there anti-aromaticity in nature?

Abstract Ab initio current density formalism is used to investigate the response to external magnetic fields of the only known naturally occurring moieties which are formally anti-aromatic, i.e., dithiines. Magnetic susceptibility, nuclear shielding constants, and the topology of induced current densities indicate that although these molecules satisfy Huckel’s rule for being anti-aromatic, they are not. In chiral dithiines, the multipolar expansion of the response contains non-vanishing anapole terms associated with ‘spinning cuff’ current lines.

Theoretical determination of magnetic properties of planar benzene isomers

Four theoretical methods, using continuous transformation of the origin of the current density (CTOCD) to annihilate either the diamagnetic or the paramagnetic contribution, have been employed at the coupled Hartree–Fock level of accuracy to evaluate magnetic susceptibility and nuclear magnetic shielding of five planar molecules with the chemical formula C6H6. The results have been compared with corresponding estimates from basis sets of London orbitals, showing the near Hartree–Fock quality of calculated values and the practicality of CTOCD approaches. Maps of streamlines and intensity of the current density induced in the π-electrons by a stationary magnetic- field perpendicular to the molecular plane are shown to interpret the different magnetic response properties of benzene isomers by means of the Ring–Current model. The plots show delocalized flow in benzene, determining its enhanced diamagnetism. Diamagnetic loops, localized in the region of carbon–carbon bonds, and a paramagnetic vortex in the vic...

Theoretical determination of the magnetic properties of 2-pyrone and 4-pyrone, and o-benzoquinone and p-benzoquinone

A series of theoretical procedures, relying on the conventional common-origin and distributed-origin approaches, and adopting extended gaugeless as well as London basis sets, have been applied at the Hartree-Fock level of accuracy to predict magnetic susceptibilities and nuclear magnetic shieldings of four molecular systems that aroused particular interest and discussion concerning their aromatic character. The theoretical results are of near Hartree-Fock quality. Comparison with experimental values for magnetic susceptibilities seems to indicate that the latter need to be revised. Analysis of calculated properties and of plots of current density induced by the magnetic field in the π-electrons provides simple and effective tools for the classification of 2- and 4-pyrones, and o- and p-benzoquinone as non-aromatic.

An efficient coupled Hartree-Fock computational scheme for parity-violating energy differences in enantiomeric molecules

Abstract A coupled Hartree–Fock scheme for fast computation of parity-violating energy differences in big-size chiral molecules of biological interest has been developed within the McWeeny–Diercksen density matrix formulation. All the required integral files and relevant matrices are represented over a basis set of atomic spin orbitals, avoiding transformation to the molecular basis. Two-electron spin–orbit matrix elements need not to be stored within the computer code implementing the theoretical method.