Split charge equilibration method with correct dissociation limits

Analytic reactive potentials rely on electronegativity equalization to describe how the electron distribution is affected as chemical reactions occur. However, such models predict fractional charges for neutral species with different electronegativities. To overcome this well-known dissociation problem, an approach taking advantage of the concept of split charges [R. A. Nistor, J. G. Polihronov, M. H. Muser, and N. J. Mosey, J. Chem. Phys. 125, 094108 (2006)] is put forward. A first implementation is presented. Starting from a previous model [P. Bultinck, W. Langenaeker, P. Lahorte, F. D. Proft, P. Geerlings, M. Waroquier, and J. P. Tollenaere, J. Phys. Chem. A 106, 7887 (2002)], a new contribution to the total energy is introduced in order to make up for the lack of suitable constraints on the charge density. Its effect is to restrain charge transfer between remote atoms. As a consequence, systems in gas phase naturally decompose into neutral fragments. This result is achieved using two empirical parameters in addition to atomic electronegativities and hardnesses.     

A generalization of the method of stimulated emission pumping

Drawing on the results of an analysis of the nature of the pulse that generates complete transfer of population from one to another level in a system with a discrete spectrum, a generalization of the method of stimulated emission pumping is proposed. It is shown that a small subset of the Fourier components of the optimal pulse will, if their relative amplitudes are the same as in the optimal pulse, generate almost as efficient a population transfer, thereby generating the opportunity to prepare a system in a selected state with a selected population.     

A complex neutron activation method for the analysis of biological materials

The aim of the present work was to deal primarily with a few essential trace elements and to obtain reliable results of adequate accuracy and precision for the analysis of biological samples. A few other than trace elements were determined by the nondestructive technique as they can be well evaluated from the gamma-spectra. In the development of the method BOWEN's kale was chosen as model material. To confirm the reliability of the method two samples were analysed proposed by the IAEA in the frame of an international comparative analysis series. The comparative analysis shows the present method to be reliable, the precision and accuracy are good.     

An analytic method for three-center nuclear attraction integrals: A generalization of the Gegenbauer addition theorem

A completely analytic method for evaluating three-center nuclear attraction integrals for STOS is presented. The method exploits a separation of the STO into an ‘evenly loaded’ solid harmonic and a OS STO . The harmonics are translated to the molecular center of mass in closed finite terms. The OS STO is translated using the Gegenbauer addition theorem; 1s STOS are translated using a single parametric differentiation of the OS formula. Explicit formulas for the integrals are presented for arbitrarily located atoms. A numerical example is given to illustrate the method.     

Recent applications and developments of charge equilibration force fields for modeling dynamical charges in classical molecular dynamics simulations

With the continuing advances in computational hardware and novel force fields constructed using quantum mechanics, the outlook for non-additive force fields is promising. Our work in the past several years has demonstrated the utility of polarizable force fields, in our hands those based on the charge equilibration formalism, for a broad range of physical and biophysical systems. We have constructed and applied polarizable force fields for small molecules, proteins, lipids, and lipid bilayers and recently have begun work on carbohydrate force fields. The latter area has been relatively untouched by force field developers with particular focus on polarizable, non-additive interaction potential models. In this review of our recent work, we discuss the formalism we have adopted for implementing the charge equilibration method for phase-dependent polarizable force fields, lipid molecules, and small-molecule carbohydrates. We discuss the methodology, related issues, and briefly discuss results from recent applications of such force fields.     

A generalization for the distance dependence of the resonance integral

The distance dependence of the resonance integral given by us earlier, has been generalized to cover bonds containing heteroatoms. The validity of the new expression has been checked for molecules containing nitrogen, oxygen, fluorine and chlorine atoms. Systems containing more than one heteroatom have also been studied.     

Determination of the charge distribution in nonmetallic crystals. I. Theory and application to tetrahedrally coordinated solids

A SCF method using localized molecular orbitals which are built up on hybrid atomic orbitals is proposed to obtain the charges in infinite crystals. Hybrid orbitals are built up on a minimal STO basis set. The formalism has been adapted in order to take into account the periodicity of the system and its infinite size by introducing the Madelung constant. The total energy is given by an infinite sum of terms each corresponding to the energy of a bond in the crystal field. Minimizing this bond energy with respect to eigenvectors it is straightforward to obtain the electronic charges, whence the polarity, i.e., the ionicity, of bonds. In this first paper, we study and discuss the polarity of bonds in zincblende and wurtzite-type compounds built up on first and second row elements. Our values are coherent between themselves and in agreement with other authors' results. The connection with electronegativity and polarizability is discussed.     

A method to reduce the equilibration time prior to data capture in ampoule-based isothermal microcalorimetric studies

This paper reports a technique for heat conduction isothermal microcalorimeters, which allows, by means of a lowering apparatus, the sample and reference ampoules to be lowered very slowly from the pre-equilibration position to the measuring position. The purpose is to minimize the effect on (total) equilibration time arising from both the pre-equilibration time and the dissipation time (i.e. the time to dissipate the thermal shock which occurs when the vessels are lowered manually). Measurements using 3 ml glass ampoules filled with water, as sample and reference, at 25 and 60 °C, showed that the extent of thermal shock was drastically diminished as the lowering speed decreased, and that the associated heat quantity was reduced to less than 0.2 mJ when lowering occurred over a period of more than 180 s. The dissipation time, the time-period required to dissipate the thermal shock, was also shortened to less than 10 min; the standard manual lowering procedure dissipation time being ca. 25 min. In practice experimental measurements of the imidazole catalysed hydrolysis of triacetin and of the solid state oxidation of ascorbic acid showed that the dissipation time was not shortened when the initial power observed was more than 5 μW, however, it was significantly shortened when the initial power observed was around ≤1 μW. The proposed ampoule lowering procedure could be expected to bring about a saving in the total measurement time for reactions with low initial power, such as those associated with long-term stability studies of relatively stable pharmaceuticals. The described procedure also eliminates operator-induced effects associated with manual lowering of ampoules. In principle the device described also would permit automated loading protocols to be developed.     

A generalization of the Rouse-Zimm model

The idea of consistently averaging the hydrodynamic interaction and its various consequences for Hookean dumbbells are reviewed. For long chains this idea can be used to generalize the Rouse-Zimm model for polymer solutions. Unlike the usual Rouse-Zimm model, the new model for steady shear flow predicts a nonzero second normal stress coefficient and shear rate dependent material functions. In the limit of long chains, the viscosity and the normal stress coefficients are universal functions of the reduced shear rate.This paper was presented at the Frühjahrstagung des Fachausschusses Polymerphysik der Deutschen Physikalischen Gesellschaft at Kaiserslautern (West Germany), March 12–14, 1986.     

A linear-scaling self-consistent generalization of the multistate empirical valence bond method for multiple excess protons in aqueous systems

An extension to the multistate empirical valence bond (MS-EVB) method is presented in this paper that is capable of treating multiple excess protons within the context of molecular-dynamics simulation. The computational cost of the method scales linearly with respect to the number of excess protons. Calculations for a 0.44 M HCl systems are carried out to illustrate the multiproton extension of the MS-EVB method. A significant decrease in the Eigen-type H(9)O(4)(+) cation is observed in the contact ion-pair configuration formed between Cl(-) and hydronium ions.     

A new method for recycling asymmetric catalysts via formation of charge transfer complexes

[reaction: see text]. A new concept for recycling asymmetric bis(oxazoline)-type catalysts is reported. The formation of charge-transfer complexes between the chiral ligand and trinitrofluorenone and their subsequent precipitation and reuse by addition of new substrate solutions is described. The efficiency of this procedure is demonstrated in a Diels-Alder reaction to reach the expected endo product as major isomer (up to 97% de and 94% ee): the catalyst was used up to 12 times without loss of either activity or selectivity.