Hund in 1926 speculated on the assignment of quantum numbers for the electronic states of diatomic molecules, basing his assignments on the correlation between the diatomic states and those of the united atom limit. Mulliken, who had earlier considered the problem of this assignment using old quantum theory, followed Hund in 1927, in the new quantum theory, correlating the states of a diatomic molecule from the united to the separated atoms. In 1932, he introduced the term orbital for the one-electron states of an atom or molecule. He was the first to seriously explore the orbitals of polyatomic molecules (1932). Burrau in 1927 made the first successful attempt to solve the Schrodinger's equation for H, and Condon (1927) assigned two electrons to Burrau's orbital to obtain an estimate of the binding energy of H2. Lennard-Jones (LJ) (1929) introduced the linear combination of atomic orbital model, and following the aufbau principle showed that this explained the paramagnetism of O2. Slater (1929) wrote many-electron wave functions as determinants of spin-orbitals, and LJ (1949) showed that with this formulation molecular orbitals (MOs) could be transformed into bond-localized functions; this provided the link to the valence bond approach and the traditional view of the chemical bond. Huckel (1930) was the first to develop a semiempirical MO model for π-electron hydrocarbons, and this was later extended by others for all-electron wave functions. Boys (1950) saw the implication of Gaussian functions for calculating the electron repulsion integrals needed for ab initio calculations, and a later approach by Kohn and coworkers (1964 and 1965) produced a density functional MO theory in which electron repulsion is calculated from the whole electron density. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

A modified form of Lindemann’s model shows that the melting points of the heavy inert gases and other effectively spherical molecular species are proportional to the depths of their diatomic potential wells. The success of the model when compared with experiment seems to rely on the almost constant value of the ratio of the fractional volume and entropy changes during fusion. The Lindemann proposal can be incorporated into an exactly treated statistical mechanical lattice model utilising expandable clusters which reproduces the solid–liquid melting phenomenon for argon with a realistic volume change and melting line.

The melting points of the heavy inert gases and of some other simple molecules show an excellent linear correlation with the depths of their diatomic potential wells, and the slope of the correlation line is in accord with Lindemann's theory of melting.

An exact calculation of the statistical mechanics of a cell model of hard-sphere isotherm behaviour is presented. Our calculation attributes the observed steps in the pressure–volume isotherms to entropic effects caused by the volume of space excluded by the hard core.

Potential functions have been derived for the bulk phases of C, Si and Ge which reproduce the lattice energies, lattice spacings, elastic constants and the main features of the phonon dispersion curves of the diamond structures. These potentials all make the diamond lattice more stable than the simple cubic, body-centred cubic and face-centred cubic structures; for Si and Ge this is the order or decreasing stability. The potentials have a form suitable for use in studies of melting, defect structures, surface properties, etc.

A review is given of the history of the properties of light and the development of the concepts to help our understanding of these properties.

Dipositronium (two electrons and two positrons) has a closer relationship to the hydrogen molecule than has often been assumed in past treatments. This article shows that appropriately modified mixtures of simple molecular orbital and Heitler–London wavefunctions are a good basis for the ground state of this species. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

The bicentenary of Dalton's paper, in which he set out his first "Table of the relative weights of the ultimate particles of gaseous and other bodies", provides us with the opportunity to reflect upon Dalton's life and achievements, to place his Atomic Theory in the context of its antecedents and to consider the views of some of his immediate successors.

Calculations based on accurate potentials have been made to search for bound levels of the lowest triplet states of hydrogen and lithium hydride, and the doublet ground states of HeH and LiHe. For the triplet state of H2, a previously reported high quality potential energy curve was employed. For the other three molecules, RCCSD(T) potentials have been calculated over a large range of interatomic distance and the results are shown to compare favourably with other accurate potentials. For each molecule the minimum reduced mass that would produce a bound level has been calculated. It is shown that triplet hydrogen is unbound for all isotopes (but only just for T2), and that HeH is also unbound for all isotopes. Triplet LiH is found to have bound levels for all commonly occurring isotopomers. Apparently for the first time, LiHe is found to have a bound level for the 6LiHe and 7LiHe isotopomers, and the energy of these levels have been determined.

CCSD(T)/aug-cc-pVQZ and CCSD(T)/aug-cc-pV5Z methods have been employed to obtain accurate interatomic potentials for Li+·He, from which spectroscopic parameters are derived. A potential is presented using the larger basis set consisting of 91 points (R⩾1.3 Å). An accurate value for the long-range D4 parameter could be obtained from the potential, but it was not possible to extract higher Dn terms. Since accurate values for Dn (n=4, 6, 7 and 8) could be derived from literature data, these parameters were fixed (and damped), allowing the short-range potential to be fitted accurately to a Born–Mayer potential.

Experiments conducted in the gas phase have led to the formation of a series of stable gold(II) complexes with nitrogen- and oxygen-containing ligands. Such complexes are very rare in condensed-phase chemistry. However, there is also a significant group of potential ligands, for example, H2O and NH3, for which stable complexes could not be formed. There are strong similarities between these observations and earlier results presented for silver(II), but both metal ions behave markedly different from copper(II). As a group the majority of successful gold(II) ligands are characterized by being good sigma donor-pi acceptor molecules; however, it is also possible to understand the ability of individual ligands to stabilize the metal ion in terms of a simple electrostatic model. Application of the latter reveals a semiquantitative trend between the physical properties of a ligand, e.g. ionization energy, dipole moment, and polarizability, and the ligand's ability to stabilize either Cu(II), Ag(II), or Au(II). The model successfully accounts for the preference of Cu(II) for aqueous chemistry, in comparison to the complete absence of such behavior on the part of Ag(II) and Au(II). Ligands from recent examples of stable condensed-phase gold(II) complexes appear to meet at least one of the criteria identified from the model.

A review of studies that have been made using the Murrell–Mottram two-plus-three-body empirical potential is presented. The explicit many-body nature of the potential is described and the fitting of these potentials to experimental data on one or more solid phases is detailed. Comparisons are made between potentials for various nonmetallic and metallic elements, from which trends in the parameters defining the potentials can clearly be seen. Examples of the many applications of these potentials to the study of solids (relative stabilities and phase transitions), surfaces (energies, relaxation and reconstructions), melting (both of the bulk and of the surfaces), and clusters (structures, growth, and dynamics) are given.

The discovery of high T-c superconducting cuprates occurred over a decade ago but the cause of the superconducting condensation and electronic structure of such compounds is still a matter of considerable debate. While there is no agreement as to the pairing mechanism, there is, on the other hand, a wide consensus about the main properties which a theoretical description should provide. In this article, a theory is presented which accounts in a straightforward way for many of the essential properties of the high T-c cuprate superconductors. Some further developments of the model are suggested, particularly relating to the normal state spin-gap which our model does not currently describe. (C) 1999 John Wiley & Sons, Inc.

There is a widely shared consensus that the pair condensate wavefunction in cuprate superconductors has a dominant dx2−y2 symmetry. In this paper a group theoretical analysis of the pair condensate wavefunction in real space for cuprate superconductors is presented. The analysis indicates that there is a degenerate pair of active bands in cuprate superconductors which are principally derived from localized e-orbitals and two possible singlet pair states formed from two different orbitals with symmetries a1, b1 or a2, b2 and all three may contribute to the condensate wavefunction. Hund's rule and the experimental observation that singlet pairing with a short coherence length occurs in cuprate superconductors, indicates that the first option is the most likely candidate in these materials and that hole doping mainly occurs from 1B1 pairs of e-orbitals giving rise to singlet electron pairing and the phenomenon of high Tc cuprate superconductivity.

The development of semi-empirical electronic structure methods from the early 1930s to the present day is reviewed. The emphasis is on the valence bond and molecular orbital treatments of molecules but some coverage of solids is given, in so far as it follows chemical models. The current position of such theories in respect of the conceptual insight they provide and their success for predictions of chemical properties is examined.

Geoffrey Dawes spent almost his entire research career as Director of the Nuffield Institute for Medical Research, which he transformed into the best–known and respected centre for research in foetal physiology. Carrying on where Sir Joseph Barcroft left off with the outbreak of the First World War, Dawes became the doyen in the field. Although his experiments were made on animals, his eye was always on human applications. His work laid the basis of many important advances in the care of the human foetus and newborn.

An empirical potential energy function, comprising two- and three-body terms, has been derived for aluminium. This potential reproduces the experimental energies and relaxations of the (111), (110) and (100) surfaces of fcc Al to a high degree of accuracy. The melting of bulk fcc Al and its low index surfaces has been studied in detail by employing Monte Carlo simulation techniques. Melting has been defined in terms of a number of calculated quantities: order parameters, density profiles and radial distribution functions. The many-body potential overestimates the bulk melting temperature (Tm ≈ 1275 ± 25 K) by approximately 37% but reproduces the sharp melting transition observed experimentally. To obtain a melting point in agreement with experiment we would need to lower the energy scaling parameter (determined from room temperature data) by 37%. The potential also indicates that the relatively open (110) surface melts about 200 K below the bulk, while the denser (100) surface does not melt until Tm. These findings are again in good agreement with experiment and with previous calculations.

It is argued that Yang's η-paried wave function is the electronic ground state of high TC Cuprate Superconductors

Classical trajectories have been carried out for the collision of helium atoms with He2+ and He3+ using a potential energy which includes the effects of dispersion, polarisation and charge exchange. At a collision energy of 0.1 eV the cross-section for atom exchange in He + He2+ is calculated to be 21.5Å, exchange occurring through both simple collisions and long-lived complexes. However, even for very long-lived complexes exchage never exceeds 50% so that complete permutation of atoms in the complex does not occur. The cross-section for exchange in He + He3+ at the same collision energy is found to be much smaller at 0.7Å2 which is attributed to the much smaller dissociation energy of He4+. Rotational energy transfer for both collisions is similar, for impact parameters less than 3.5Åonly 50% of collisions transfer more than 10% of translational energy to rotation.