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This paper assesses the utility of momentum space in the density-based quantification of the steric effect proposed by Liu [J. Chem. Phys. 126, 244103 (2007)], which is based on a new energy partition scheme where the total electronic energy is decomposed into contributions from three independent effects: steric, electrostatic, and the fermionic quantum. The steric energy defined in this way is repulsive, exclusive, and extensive and intrinsically linked to Bader's quantum theory of atoms in molecules. In this work, the plausibility of defining and computing steric energies using momentum densities according to this scheme is confirmed from numerical tests. Moreover, we found that the correlation between the experimental scales of the steric energies and theoretical values computed from momentum densities is even better than the same correlation with theoretical estimations obtained from position densities. [ABSTRACT FROM AUTHOR]

We present a time-dependent method for determining several approximate excited-state energies and wave functions using a vectorial approach to multidimensional supersymmetric quantum mechanics. First, a vectorial approach is used to generate the tensor sector two Hamiltonian, which is isospectral with the original scalar sector one Hamiltonian above the ground state of the sector one Hamiltonian. We construct a time-dependent Hamiltonian interpolating between the scalar sector one Hamiltonian and the tensor sector two Hamiltonian. Then, we can adiabatically switch from the ground state of the sector one Hamiltonian to the ground state of the sector two Hamiltonian by solving the time-dependent Schrödinger equation. In addition, by employing an initial wave packet orthogonal to that leading to the ground state of sector two, we also obtain the first-excited state of sector two. Construction of the orthogonal sector one states is trivial due to the tensor nature of sector two. The ground and first-excited states of the sector two Hamiltonian can be used with the charge operator to obtain the first two excited state wave functions of the sector one Hamiltonian. Excellent computational results are obtained for two-dimensional nonseparable degenerate and nondegenerate systems. [ABSTRACT FROM AUTHOR]

An overview is presented of the notions of size-extensivity and size-consistency, which play an important role in the discussion of many-body methods in physics and chemistry. We also introduce the concept of generalized extensivity, which has been used before implicitly in the literature, and provide an operational definition, which can be tested numerically in a similar way as size-consistency. A numerical example illustrates the concept of generalized extensivity as applied to the EOM-CCSD, STEOM-CCSD and extended-STEOM-CCSD electronic structure methods for excited states. In another line of thought it is argued that algebraic proofs are often more easily constructed than diagrammatic proofs to demonstrate proper separation into non-interacting fragments. The algebraic line of reasoning is illustrated for single reference methods and applied to discuss separability properties for a variety of Hilbert space multi-reference coupled cluster methods. [ABSTRACT FROM AUTHOR]

The first four spectroscopic states of the silylene molecule SiH 2 , namely, and were examined theoretically using multireference methods coupled with very large correlation consistent basis sets. Our aim is understanding why SiH 2 has a singlet ground state ( ) as opposed to the state of the isovalent carbene CH 2 , as well as the rationalization of its geometric and bonding characteristics. The interpretational philosophy followed is based on strictly calculable quantities in an effort to reduce to a minimum the always present but not well-defined 'chemical intuitionism'. All of our calculated quantities are in excellent agreement with existing experimental results. [ABSTRACT FROM AUTHOR]

In this combination of review and new science, we shall look at Density Functional Theory (DFT) and the Quantum Monte Carlo (QMC) method. For DFT, we shall (i) discuss the inadequacy of the Uniform Electron Gas model for chemistry (ii) examine Generalized Gradient Approximation (GGA) exchange functionals (iii) understand that they introduce left-right correlation for molecules (iv) argue that the functionals BLYP and OLYP are parameter-free for molecules (v) look at the 'remarkable' performance of these local functionals in the prediction of binding energies (vii) discuss hybrid functionals, and show how they can be improved through the introduction of an Ewald partitioning of 1/ r 12 , already examined by Hirao, Savin and Gill (viii) show that our new functional CAM-B3LYP offers good energetics as well as good charge transfer excitation energies, already shown by Hirao. For QMC, concentrating on the Diffusion Monte Carlo approach, we shall (i) relate the trial wavefunction to the transcorrelated wavefunction, and look again at the transcorrelated method, (ii) look at the nodal surfaces of the trial wavefunction, demonstrating in particular that the use of finite basis sets in the Hartree-Fock method necessarily introduces 'spurious' nodal features which probably cannot be ignored, (iii) argue that, now it is possible to perform Hartree-Fock calculations with slater-type basis sets, it is straightforwardly possible to obtain orbitals which obey the exact nuclear cusp conditions, (iv) therefore argue that these basis sets should be used in Monte Carlo studies. [ABSTRACT FROM AUTHOR]

Two new classes of non-iterative corrections to the ground- and excited-state energies obtained in the state-universal multi-reference coupled-cluster (SUMRCC) calculations have been developed using the multi-reference extension of the method of moments of coupled-cluster equations (MMCC) [K OWALSKI , K., and P IECUCH , P., 2001, J. molec. Struct. (THEOCHEM) , 547, 191]. In the first class of the configuration interaction (CI) corrected multi-reference MMCC (MRMMCC) approximations, the non-iterative corrections due to triply or triply and quadruply excited clusters are constructed with the help of multi-reference CI (MRCI) calculations employing the same active space as used in the SUMRCC calculations. In the second class of the completely renormalized (CR) SUMRCC methods, which can be viewed as the multi-reference extensions of the single-reference CR-CCSD(T) theory [K OWALSKI , K., and P IECUCH , P., 2000, J. chem. Phys. , 113, 18], the non-iterative corrections due to triply excited clusters are constructed with the help of the multi-reference many-body perturbation theory. In both cases, the non-iterative corrections due to higher-order clusters are added to the energies obtained with the SUMRCC method with singles and doubles. It is demonstrated that the newly developed corrections, including the CR-SUMRCC methods, offer considerable improvements in the SUMRCCSD results, reducing, in particular, the large errors in the SUMRCCSD results due to intruders. [ABSTRACT FROM AUTHOR]

Potential energy curves of ground and low-lying singlet states of Be 2 were obtained using the multi-configuration quasi-degenerate perturbation theory. The effects of a cylindrical harmonic confining potential on the spectroscopic parameters of these states were studied. The equilibrium internuclear separation decreases with the strength of the potential and all electronic states become more strongly bound when the potential is present. For both the states, a switch between dissociation limits is observed from 1 S + 1 P to 3 S + 3 P due to re-ordering of the molecular orbitals. Electron affinities of Be 2 are also calculated and it is found that the extra electron is not bound to Be 2 when the molecule is enclosed in the potential. [ABSTRACT FROM AUTHOR]

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17 O- 17 O and 15 N- 15 N spin-spin coupling constants ( 2h J O-O and 2h J N-N ) for ten neutral structures which may exhibit intramolecular O-H-O or N-H-N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin-spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O-O distance and stronger hydrogen bond in the former. However, when the O-O or N-N distances are forced to be the same in corresponding saturated and unsaturated structures, the coupling constants are similar. There is a significant increase in coupling constants when the intramolecular hydrogen bond (IMHB) changes from asymmetric to symmetric, due to much shorter O-O or N-N distances . Symmetrization effects also significantly affect the value of the proton chemical shift. For hydrogen-bonded conformers the trends in chemical shifts and coupling constants are similar. A detailed analysis of the NMR properties of oxygen-containing systems leads to the conclusion that neither the coupling constants nor the proton chemical shifts provide any evidence for the existence of RAHBs. Furthermore, the strength of the O-H···O IMHBs, investigated through the use of appropriate homodesmotic reactions, indicates that the enhanced stability of the IMHB in the unsaturated compound is associated with the σ skeleton of the molecule that allows the oxygen atoms to be in closer proximity than in the saturated analogue. [ABSTRACT FROM AUTHOR]

We present an efficient, parallel implementation for the calculation of Hartree-Fock and density functional theory analytical Hessian (force constant, nuclear second derivative) matrices. These are important for the determination of harmonic vibrational frequencies, and to classify stationary points on potential energy surfaces. Our program is designed for modest parallelism (4-16 CPUs) as exemplified by our standard eight-processor QuantumCube™. We can routinely handle systems with up to 100+ atoms and 1000+ basis functions using under 0.5 GB of RAM memory per CPU. Timings are presented for several systems, ranging in size from aspirin (C 9 H 8 O 4 ) to nickel octaethylporphyrin (C 36 H 44 N 4 Ni). [ABSTRACT FROM AUTHOR]

The results of an ab initio study of the magnetic hyperfine structure of the X 2 Π electronic state of HCCS and DCCS are reported. The potential surfaces for two components of the X 2 Π electronic state and the electronically averaged hyperfine coupling constants for H, D, 13 C and 33 S are obtained as functions of two bending vibrational modes by the density functional theory method. Since the bending potential curves calculated in the present study do not differ significantly from their counterparts computed in our previous work by means of an extensive configuration interaction approach, we employ the latter here to avoid the appearance of small differences between the vibronic energy levels presented in this work and those already published. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. It is found that, due to the generally significant geometry dependence of the hyperfine coupling constants, it is necessary to carry out vibronic averaging of the corresponding functions in order to obtain values which can be compared with the results of measurements. The results of the present study are an aid to the reliable interpretation of recently published experimental data. [ABSTRACT FROM AUTHOR]

A simple variational procedure, termed DOPI for discrete variable representation--Hamiltonian in orthogonal coordinates--direct product basis-iterative diagonalization, is described and applied to compute low-lying vibrational band origins (VBOs) of the triatomic systems H 2 O, CO 2 , and N 2 O, employing published empirical and theoretical sextic force fields. While in these cases no difficulties arise when quartic potentials are used, the limited range of applicability of 6th-order potentials presents difficulties for the variational determination of VBOs, in particular for the higher-lying bending states. For H 2 O, transformation of quadratic and quartic force fields from simple bond stretching to Simons-Parr-Finlan (SPF) coordinates results in computed VBOs deviating less from experiment. This, however, does not hold for the VBOs computed from the transformed sextic force fields where the two representations provide highly similar results. While use of empirical quartic and sextic force fields result in a much better reproduction of experimental VBOs than that of ab initio force fields, especially at higher (fifth- and sixth-) order the empirical force constants, obtained through different refinement procedures, do not correspond to the associated derivatives of the potential energy surface (PES). Rotational constants characterizing low-lying vibrational states have been evaluated as expectation values using inertia tensor formulas in the Eckart and principal axis frames. Only the Eckart axes should be used for these computations and they yield accurate vibrationally averaged rotational constants. [ABSTRACT FROM AUTHOR]

It is argued that awareness of the distinction between dynamical and variational symmetrics is crucial to understanding the significance of Noether's 1918 work. Special attention is paid, by way of a number of striking examples, to Noether's first theorem, which establishes a correlation between dynamical symmetries and conservation principles. [ABSTRACT FROM AUTHOR]

Using augmented polarized correlation consistent basis sets extended with midbond functions, we evaluate the ground state interaction potential and the induced electric dipole polarizabilities and first and second hyperpolarizabilities of the He-Ar, Ne-Ar and He-Ne van der Waals complexes. For the calculation of the potential we resort to the coupled cluster singles and doubles (CCSD) model corrected for triple excitations, CCSD(T), whereas properties are evaluated with CCSD response theory. As a check of the quality of the potential, the rovibrational spectrum and the gas second virial coefficients are evaluated. The rovibrational spectra improve previously available theoretical results, although the dissociation energies are probably still slightly underestimated. For the gas second virial coefficients the agreement with experiment is satisfactory. The frequency dependence of the interaction (hyper)polarizabilities is analysed and a comparison with previous results on the mixed dimers and the pure gases is made. [ABSTRACT FROM AUTHOR]

A two-electron effective operator V 2 is constructed to reproduce, to lowest order of perturbation theory, the effect of p-electron admixtures in configurations of f electrons occurring in rare-earth ions located at octahedral sites in elpasolite crystals. These admixtures can arise from excitations of the type f → p as well as p → f, the latter including electron transfer from neighbouring ligands to the central rare-earth ion. The effective operator is decomposed into nine orthogonal components . For octahedral symmetry their strengths are specified by coefficients that depend on just two parameters. The insensitivity of the crystal splitting of of to some , as found by McCaw and Denning, is related to exceptionally small values of the corresponding reduced matrix elements of . The discrepancies that these authors have found in fitting their experimental data for other levels to the standard one-electron crystal-field Hamiltonian V 1 are examined. The effective operator V 2 is found to reproduce the general trends of these discrepancies provided allowance is made for transitions of the type f ← f ′ which are implicit in the spin-correlated crystal-field model. [ABSTRACT FROM AUTHOR]

Formulas are derived for consistent evaluation of permutationally totally symmetric and determinantally antisymmetric powers of generalized combinations in which irreducible representations may occur with either positive or negative weights. [ABSTRACT FROM AUTHOR]

The frequency and temperature dependence of real (ε′) and imaginary (ε′′) parts of the dielectric constant of polycrystalline complexes (β-CD) 2 ·LiI 7 ·8H 2 O and (β-CD) 2 ·KI 7 ·8H 2 O (β-CD = β-cyclodextrin) have been investigated over the frequency and temperature ranges of 0-100 kHz and 120-300 K. The temperature dependence of ε′, ε′′ and phase shift ϕ showing two steps, two peaks and two minima respectively, reveals the existence of two kinds of water molecule, the tightly bound and the easily movable water molecules, in both complexes. The first peak of ε′′( T ) or the first minimum of φ( T ) present the transformation of flip-flop hydrogen bonds to the normal state. The second ε′′( T ) peak or φ( T ) minimum correspond to the easily movable water molecules or to a partial transformation of tightly bound to easily movable water molecules. Both samples for T >275 K show semiconductive behaviour with energy gaps 0.72 eV for the (β-CD) 2 ·LiI 7 ·8H 2 O and 0.58 eV for the (β-CD) 2 ·KI 7 ·8H 2 O complex. The conductivity at temperatures T >220 K is greater for the Li complex and at T < 220 K both complexes have similar conductivity values. The relaxation time varies in a Λ-like curve (from 180 K to 260 K) and rises rapidly for temperatures greater than 260 K, indicating the process of ionic movements. The activation energies around the transition temperature 0.40-0.50 k B T trans for the (β-CD) 2 ·LiI 7 ·8H 2 O and 0.78-1.00 k B T trans for the (β-CD) 2 ·KI 7 ·8H 2 O reveal the greater stability of the β-K complex against that of the β-Li complex. [ABSTRACT FROM AUTHOR]

In order to explain the adsorption isotherms of the amphiphilic penicillins nafcillin and cloxacillin onto human serum albumin (HSA), a cooperative multilayer adsorption model is introduced, combining the Brunauer-Emmet-Teller (BET) adsorption isotherm with an amphiphilic ionic adsorbate, whose chemical potential is derived from Guggenheim's theory. The non-cooperative model has been previously proved to qualitatively predict the measured adsorption maxima of these drugs [V arela , L. M., G arcía , M., P érez -R odríguez , M., T aboada , P., R uso , J. M., and M osquera , V., 2001, J. chem. Phys. , 114, 7682]. The surface interactions among adsorbed drug molecules are modelled in a mean-field fashion, so the chemical potential of the adsorbate is assumed to include a term proportional to the surface coverage, the constant of proportionality being the lateral interaction energy between bound molecules. The interaction energies obtained from the empirical binding isotherms are of the order of tenths of the thermal energy, therefore suggesting the principal role of van der Waals forces in the binding process. [ABSTRACT FROM AUTHOR]

Continuous wave and pulsed X-band electron paramagnetic resonance experiments have been performed for Cu 2+ ions substituting Zn(II) in Ba 2 Zn(HCOO) 6 ·4H 2 O single crystals in the temperature range 4.2-300 K. Spin-Hamiltonian parameters of octahedral Cu(HCOO) 2 (H 2 O) 2 complexes at the rigid lattice limit (below 77 K) are the following: g z = 2.405, g y = 2.095, g x = 2.065, A z = 130, A y = 4, and A x = 11 × 10 -4 cm -1 . The parameters along the y and z -axes display vibronic averaging effects on heating which allows evaluation of the energy difference δ 12 = 347 cm -1 between the two lowest energy Jahn-Teller distorted configurations. The electron spin-lattice relaxation is governed by Raman processes and the Debye temperature has been determined as Θ D = 204 K. The electron spin echo amplitude decay is determined by instantaneous diffusion below 10 K and then the dephasing rate weakly increases on heating because of libration motions of Cu 2+ complexes and/or excitations to the first excited vibronic level of energy Δ = 63 cm -1 . [ABSTRACT FROM AUTHOR]

The mean-field ENMO (electronic and nuclear molecular orbitals) wave function is written as a product of one-particle functions that pertain to different types of particles, antisymmetrized over the fermionic orbitals, thus allowing the simultaneous treatment of electrons and nuclei within a self-consistent framework. We implemented the ENMO-SCF (self-consistent field) as well as correlated versions of ENMO analogous to the MBPT (many-body perturbation theory) and CI (configuration integration) models of Born-Oppenheimer quantum chemistry. The ENMO methods are attractive by virtue of their similarity to traditional electronic structure theory models, and several researchers have recently implemented such approaches. Here we present the first fundamental study of the convergence of ENMO methods with respect to basis set and correlation treatment for simple systems. It is found that the elimination of the translational invariance of the full molecular Hamiltonian is pivotal for obtaining a physically meaningful spectrum. For example, the ENMO-full CI spectrum for the hydrogen atom contains a manifold of spurious states which cannot be removed by simple subtraction of centre-of-mass kinetic energy because of the general non-factorizability (non-separability) of the ENMO ansatz into internal and centre-of-mass parts. An alternative, hybrid approach that treats only some light nuclei in a molecule quantum-mechanically avoids the spurious states problem by breaking the translational invariance of the Hamiltonian. However, both simple analytical estimation and numerical examples for some diatomic hydrides show that vibrational energy levels are not accurately predicted in this approach; this suggests difficulty in using such methods to predict tunnelling splittings. Finally, slow convergence with respect to basis set and correlation method is observed and related to the failure of ENMO methods to describe interparticle cusps (including the electron/nuclear cusp) efficiently due to the lack of explicit interparticle coordinates. We elevate these previously unrecognized or underappreciated difficulties as being of prime importance to future progress in ENMO methods. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations of binary mixtures of benzene, 1,3,5-trifluorobenzene and hexafluorobenzene with dimethylimidazolium hexafluorophosphate were carried out to examine their macroscopic and microscopic properties. The energies and volumes of mixing of these mixtures correlate well with observed microscopic properties including coordination number about the aromatic compound. The local ordering of the ions about an aromatic molecule was found to depend on the quadrupole moment of the aromatic species and to remain qualitatively the same on varying the mole fraction of the aromatic species. Interaction energies showed the most significant interactions to be between the aromatic molecule and the ions located about its equator. These findings have implications for the practical use of ionic liquids as solvents for chemical processes. [ABSTRACT FROM AUTHOR]

We report static electric (hyper)polarizability values of the three linear isomers of (CN) 2 . Large, flexible basis sets have been especially designed for all three molecules and the electric properties have been obtained from finite-field Møller-Plesset perturbation theory and coupled cluster calculations. All three molecules are characterized by large polarizability anisotropies. Self-consistent field calculations of near-Hartree-Fock quality yield the following mean polarizabilities: for CN-NC, 32.42 for NC-CN and 33.46 for CN-CN. For the second hyperpolarizability we obtain for CN-NC, 3090 for NC-CN and 3748 for CN-CN. Thus, electric (hyper)polarizability increases in the order NC-CN

Using the mean spherical approximation, an explicit analytical equation of state with non-dimensional variables for one-component multi-Yukawa fluids is established based on the work of Blum and Ubriaco. Two simplified calculation methods (one is the linear summation method and the other is the negligible entropy method) are proposed. The compressibility factors for one-component 1-4 Yukawa fluids are simulated with the Monte-Carlo method. Comparisons between various methods under different range parameters are listed and discussed for 1-4 Yukawa fluids. These indicate that our methods are reliable for a wide range of parameters and can be used conveniently. [ABSTRACT FROM AUTHOR]

Thermodynamic perturbation theory is used to calculate the free energies and resulting phase diagrams of binary systems of spherical colloidal particles and interacting polymer coils in a good solvent within an effective one-component representation of such mixtures, whereby the colloidal particles interact via a polymer-induced depletion potential. Monte Carlo simulations are used to test the convergence of the high temperature expansion of the free energy. The phase diagrams calculated for several polymer-to-colloid size ratios differ considerably from the results of similar calculations for mixtures of colloids and ideal (non-interacting) polymers, and are in good overall agreement with the results of an explicit two-component representation of the same system, which includes more than two-body depletion forces. [ABSTRACT FROM AUTHOR]

A weakly bound linear complex of N 2 and HHeF was found to be stable with respect to the constituent monomers by ab initio calculations at various levels of theory (MP2, MP3, MP4(SDQ) and QCISD) using a 6-311++G(2d,2p) basis set. The complex N 2 ···HHeF was found to have a zero-point vibrational energy corrected binding energy of 14.5 kJ mol -1 (QCISD) and exhibits a large harmonic vibrational frequency blue shift of 375 cm -1 for the He-H stretching vibration mode, with a diminished infrared intensity for this mode on formation of the complex. The frequency shift for this mode was also found to be very sensitive to the level of theory employed for the calculation, and is rationalized by considering intermolecular electrostatic and charge-transfer effects. The results for N 2 ···HHeF are compared with corresponding results for the related complexes N 2 ···HArF and N 2 ···HKrF, both of which contain the same proton acceptor molecule. [ABSTRACT FROM AUTHOR]

We consider propagation of a linearly polarized, harmonic, plane electromagnetic wave incident on a dilute gas subject to a non-uniform electrostatic field. We work to electric quadrupole-magnetic dipole order. In general, the wave is distorted to include a perpendicular component induced by the electrostatic field gradient, and the resultant is not a plane wave. Although the induced component is small, its contribution to the polarization-dependent refractive index of the gas in Buckingham's quadrupole experiment is significant. We show that distortion of the incident wave accounts exactly for the present discrepancy between wave theory and forward scattering theory of electric-field-gradient-induced birefringence in a gas: both theories yield the result obtained originally by Buckingham and Longuet-Higgins. [ABSTRACT FROM AUTHOR]

The electronic transition C 1 Σ + -A 1 Π has been observed for the first time for CH + and CD + . The n -4 and ( n + 1)-5 bands have been rotationally analysed for CH + and CD + . [ABSTRACT FROM AUTHOR]

The reaction of the iron oxide cation with the hydrogen molecule is studied in this work using the diffusion quantum Monte Carlo (DMC) method. Previous attempts achieved a qualitative agreement between theory and experiments, but not a satisfactory quantitative explanation which concurs with the experimental facts. The DMC method provides a consistent explanation of the experimental results all along the reaction path. This is due to the fact that DMC accounts for almost all of the correlation energy, which is of vital importance for the correct description of this reaction. [ABSTRACT FROM AUTHOR]

The author discusses the role of the physicists in advancing the research and study on molecular physics. He presents the 50th anniversary of the periodical "Molecular Physics," followed by discussion on the contribution of the periodical to chemistry and physics. The author argues that the concerns and achievements of the journal's founders are valid and relevant today.

Comments on research articles on molecular physics published in the June 2005 issue of the "Molecular Physics" periodical. Background information on Professor John Simons, to whom the special issue was dedicated; Analysis of pertinent topics and relevant issues; Implications on studies of molecular physics.

Announces a change in reference style for the journal "Molecular Physics." Decision to give all references a separate number within the text to make these articles fully searchable and improve cross referencing between different journals and publishers.