Long range intermolecular forces in triatomic systems: connecting the atom-diatom and atom-atom-atom representations

The long-range forces that act between three atoms are analysed in both atom-diatom and atom-atom-atom representations. Expressions for atom-diatom dispersion coefficients are obtained in terms of 3-body nonadditive coefficients. The anisotropy of atom-diatom C_6 dispersion coefficients arises primarily from nonadditive triple-dipole and quadruple-dipole forces, while pairwise-additive forces and nonadditive triple-dipole and dipole-dipole-quadrupole forces contribute significantly to atom-diatom C_8 coefficients. The resulting expressions are applied to dispersion coefficients for Li + Li_2 (triplet) and recommendations are made for the best way to obtain global triatomic potentials that dissociate correctly both to three separated atoms and to an atom and a diatomic molecule.Comment: To be published in a special issue of Molecular Physics in honour of Mark Chil

Tunneling splittings of vibrationally excited states using general instanton paths

A multidimensional semiclassical method for calculating tunneling splittings in vibrationally excited states of molecules using Cartesian coordinates is developed. It is an extension of the theory by Mil'nikov and Nakamura [$\textit{ J. Chem. Phys.}$ $\textbf{122}$, $124311$ $(2005)$] to asymmetric paths that are necessary for calculating tunneling splitting patterns in multi-well systems, such as water clusters. Additionally, new terms are introduced in the description of the semiclassical wavefunction that drastically improve the splitting estimates for certain systems. The method is based on the instanton theory and builds the semiclassical wavefunction of the vibrationally excited states from the ground-state instanton wavefunction along the minimum action path and its harmonic neighborhood. The splittings of excited states are thus obtained at a negligible added numerical effort. The cost is concentrated, as for the ground-state splittings, in the instanton path optimization and the hessian evaluation along the path. The method can thus be applied without modification to many mid-sized molecules in full dimensionality and in combination with on-the-fly evaluation of electronic potentials. The tests were performed on several model potentials and on the water dimer.Comment: The following article has been submitted to Journal of Chemical Physics. After it is published, it will be found at https://aip.scitation.org/journal/jc

A New Approach to Stoichiometric Problem Solving

Predlaže se da se stehiometrijski računi temelje na promjenama mjerenih veličina pojedinih sudionika kemijske reakcije kao što su mase, množine, broj molekula itd. te da se odgovarajuće podesi označivanje: da se promjene veličina označuju operatorom za razliku Δ kao što je uobičajeno u termodinamici. Umjesto omjera masa, množina ili drugih veličina bolje je pisati omjere promjena tih veličina. Pritom se kao posljedica nužno javljaju i negativne vrijednosti koje se automatski kompenziraju prihvaćenom definicijom stehiometrijskog broja. Sve su te promjene međusobno povezane preko dosega kemijske reakcije i uvodna kemija za studente kemije i srodnih struka ne bi smjela izbjegavati pojam dosega. Ta je veličina, kao "veličina stanja" napredovanja kemijskog procesa, nužna za razumijevanje kemijske termodinamike i kemijske kinetike, ali i u složenijim stehiometrijskim računima ima određene prednosti.T. Cvitaš A New Approachz to Stoichiometric Problem Soloving Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia E-mail: [email protected] It is proposed that stoichiometric problem solving be based on changes of measured physical quantities such as masses, chemical amounts, numbers of molecules, etc. of reactants or products and that the notation be adjusted accordingly: the changes should be denoted by the difference operator as is usual in the field of thermodynamics. Instead of using ratios of masses, amounts, etc. it is better to use ratios of changes in the corresponding quantities. This leads to the appearance of negative values which are automatically compensated by the accepted definition of stoichiometric numbers. All these changes are mutually related via the extent of reaction and introductory chemistry courses for chemists and related fields should not avoid the concept of extent of reaction. This quantity is, as a "quantity of state" of advancement of the chemical process, essential for the understanding of chemical thermodynamics and chemical kinetics, but it also shows certain advantages in solving more complicated stoichiometric problems