Practical failures from the inclusion of exact exchange: how much exact exchange is appropriate?

The influence of exact exchange incorporated into exchange—correlation functionals on the predictions of relative energies, structures, electronic states, and vibrational spectra is examined numerically. Failures of widely used hybrid exchange—correlation functionals due to either the physical unacceptability of including exact exchange or an unbalanced mixing of exact exchange are considered. One set of examples involves tetraatomic chalcogen clusters and charge transfer complexes between diatomic chalcogens and diatomic oxygen. Poor energetic predictions from Hartree-Fock rule against the inclusion of exact exchange into the exchange—correlation functionals for these systems with significant left—right electron correlation effects. The energies of the conformers of [10]annulene are considered from an unusual viewpoint, namely, the empirical adjustment of the admixture of exact exchange to match the predictions of very high level theoretical methods. For this annulene with insignificant left-right electron correlation effects, a greater (50%) percentage of exact exchange should be included. The relationship of symmetry breaking to the inclusion of exact exchange is examined for seven linear radicals, OXO (X = B, Al, Ga, In, TI). AIOS, and OAIS. exchange—correlation functionals generate symmetry adapted solutions at the expense of an unusual ordering of the Kohn-Sham orbitals, which can cause uncharacteristic electronic states, incorrect vibrational spectra, and poorer predictions of energetics. These effects are greater when exact exchange is included. In all the examples considered, the appropriate focus for a detailed discussion of molecular properties involves consideration of the effects of exact exchange.     

Neurotransmitters in the gas phase: a computational and spectroscopic study of noradrenaline

The conformational structures of noradrenaline, isolated in the gas phase, have been explored through a combination of electronic structure computation (at the B3LYP/6-31 + G?, MP2/6-31 + G?, MP2/aug-cc-pVDZ and CIS/6-31 + G? levels of theory) and mass selected ultraviolet and infrared ion dip spectroscopy (following laser ablation of the neurotransmitter into a pulsed supersonic argon expansion). Despite the many possible low-lying conformational possibilities predicted by theory, almost the entire population of jet-cooled noradrenaline adopts the global minimum structure, associated with an extended, AG1a, ethanolamine side chain conformation. Intramolecular hydrogen bonds are formed between the neighbouring hydroxyl groups on the catechol ring and between the hydroxyl and amino groups on the side chain.     

Vapour—liquid equilibrium of the square-well fluid of variable range via a hybrid simulation approach

The equilibrium between vapour and liquid in a square-well system has been determined by a hybrid simulation approach combining chemical potentials calculated via the Gibbs ensemble Monte Carlo technique with pressures calculated by the standard NVT Monte Carlo method. The phase equilibrium was determined from the thermodynamic conditions of equality of pressure and chemical potential between the two phases. The results of this hybrid approach were tested by independent NPT and μPT calculations and are shown to be of much higher accuracy than those of conventional GEMC simulations. The coexistence curves, vapour pressures and critical points were determined for SW systems of interaction ranges λ = 1.25, 1.5, 1.75 and 2. The new results show a systematic dependence on the range λ, in agreement with results from perturbation theory where previous work had shown more erratic behaviour.     

In Memoriam Vladimir Prelog (1906–1998): Some Personal Reminiscences

Some persons, by virtue of the classic status of their achievements and the lengthy duration of their careers, become legends in their own time and are often incorrectly believed to be no longer with us. As a case in point, in the early 1980s Charles Scribners Sons invited me to write an article on the American organic chemist Carl Shipp (Speed) Marvel for its Dictionary of Scientific Biography. Because one of the criteria for inclusion in DSB was that the subject be deceased, I informed the publisher that Marvel was very much alive (After Marvel had died at the age of 93 in 1988, I wrote the previously requested DSB article).     

Solution Branches for Non-linear Equilibrium Problems--Bifurcation and Domain Perturbations

Consider the following simple, but typical, example of a non-linearequilibrium (differential equation) problem: u = –f(u) for (x, y) D, u = 0 for (x, y) D. Usually the eigenvalues of the (small u) linearized problemare simple, and each simple eigen value generates two solutionbranches for the full problem. However, the full problem nearlyalways has many other solution branches, and this paper describeshow to find these other branches and why they arise.