Showing papers in "Journal of Chemical Physics in 1991"

Gaussian-2 theory for molecular energies of first- and second-row compounds

Abstract: The Gaussian‐2 theoretical procedure (G2 theory), based on a b i n i t i o molecular orbital theory, for calculation of molecular energies (atomization energies, ionization potentials,electron affinities, and proton affinities) of compounds containing first‐ (Li–F) and second‐row atoms (Na–Cl) is presented. This new theoretical procedure adds three features to G1 theory [J. Chem. Phys. 9 0, 5622 (1989)] including a correction for nonadditivity of diffuse‐s p and 2d f basis set extensions, a basis set extension containing a third d function on nonhydrogen and a second p function on hydrogen atoms, and a modification of the higher level correction. G2 theory is a significant improvement over G1 theory because it eliminates a number of deficiencies present in G1 theory. Of particular importance is the improvement in atomization energies of ionic molecules such as LiF and hydrides such as C2H6, NH3, N2H4, H2O2, and CH3SH. The average absolute deviation from experiment of atomization energies of 39 first‐row compounds is reduced from 1.42 to 0.92 kcal/mol. In addition, G2 theory gives improved performance for hypervalent species and electron affinities of second‐row species (the average deviation from experiment of electron affinities of second‐row species is reduced from 1.94 to 1.08 kcal/mol). Finally, G2 atomization energies for another 43 molecules, not previously studied with G1 theory, many of which have uncertain experimental data, are presented and differences with experiment are assessed.

A complete basis set model chemistry. II. Open‐shell systems and the total energies of the first‐row atoms

Abstract: The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. An open‐shell complete basis set (CBS) model chemistry, based on the unrestricted Hartree–Fock (UHF) zero‐order wave function, is defined to include corrections for basis set truncation errors. The total correlation energy for the first‐row atoms is calculated using the unrestricted Mo/ller–Plesset perturbation theory, the quadratic configuration interaction (QCI) method, and the CBS extrapolation. The correlation energies of the atoms He, Li, Be, B, C, N, O, F, and Ne, calculated using atomic pair natural orbital (APNO) basis sets, vary from 85.1% to 95.5% of the experimental correlation energies. However, extrapolation using the asymptotic convergence of the pair natural orbital expansions retrieves from 99.3% to 100.6% of the experimental correlation energies for these atoms. The total extrapolated energies (ESCF+Ecorrelation) are then in agreement with experiment to within ±0...

Theoretical foundations of dynamical Monte Carlo simulations

Abstract: Monte Carlo methods are utilized as computational tools in many areas of chemical physics. In this paper, we present the theoretical basis for a dynamical Monte Carlo method in terms of the theory of Poisson processes. We show that if: (1) a ‘‘dynamical hierarchy’’ of transition probabilities is created which also satisfy the detailed‐balance criterion; (2) time increments upon successful events are calculated appropriately; and (3) the effective independence of various events comprising the system can be achieved, then Monte Carlo methods may be utilized to simulate the Poisson process and both static and dynamic properties of model Hamiltonian systems may be obtained and interpreted consistently.

Improved algorithms for reaction path following: Higher‐order implicit algorithms

Abstract: Eight new algorithms for reaction path following are presented, ranging from third order to sixth order. Like the second‐order algorithm [J. Chem. Phys. 90, 2154 (1989)] these are implicit methods, i.e., they rely on the tangent (and in some cases the curvature) at the endpoint of the step. The tangent (and the curvature, if needed) are obtained by a constrained optimization using only the gradient. At most, only one Hessian calculation is needed per step along the path. The various methods are applied to the Muller–Brown surface and to a new surface whose reaction path is known analytically to test their ability to follow the reaction path and to reproduce the curvature along the path.

Wave functions with terms linear in the interelectronic coordinates to take care of the correlation cusp. I. General theory

Abstract: The matrix elements needed in a CI‐SD, CEPA, MP2, or MP3 calculation with linear r12‐dependent terms for closed‐shell states are derived, both exactly and in a consistent approximate way. The standard approximation B guarantees that in the atomic case the error due to truncation of the basis at some angular momentum quantum number L goes as ∼L−7, at variance with L−3 in conventional calculations (without r12 terms). Another standard approximation A has errors ∼L−5, but is simpler and—for moderate basis sets—somewhat better balanced. The explicit expressions for Mo/ller–Plesset perturbation theory of second and third order with linear r12 terms (MP2‐R12 and MP3‐R12, respectively) are explicitly given in the two standard approximations.

Optical properties of disordered molecular aggregates: a numerical study

Abstract: We present results of numerical simulations on optical properties of linear molecular aggregates with diagonal and off‐diagonal disorder. In contrast to previous studies, we introduce off‐diagonal disorder indirectly through Gaussian randomness in the molecular positions; this results in a strongly asymmetric distribution for the interactions. Moreover, we do not restrict to nearest‐neighbor interactions. We simultaneously focus on several optical observables (absorption linewidth and line shift and superradiant behavior) and on the density and the localization behavior of the eigenstates (Frenkel excitons). The dependence of these optical properties on the disorder is investigated and expressed in terms of simple power laws. For off‐diagonal disorder, such a study has not been performed before. In the case of diagonal disorder, we show that, in particular, the superradiant decay rate of the aggregates may be strongly affected by the inclusion of non‐nearest‐neighbor interactions. Recent results of absorp...

A complete basis set model chemistry. III. The complete basis set‐quadratic configuration interaction family of methods

Abstract: The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. A family of complete basis set (CBS) quadratic CI (QCI) model chemistries is defined to include corrections for basis set truncation errors. These models use basis sets ranging from the small 6‐31 G°° double zeta plus polarization (DZ+P) size basis set to the very large (14s9p4d2f,6s3p1d)/[6s6p3d2f,4s2p1d] atomic pair natural orbital basis set. When the calculated energies are compared with the experimental energies of the first‐row atoms and ions and the first‐row diatomics and hydrides H2, LiH, Li2, CH4, NH3, H2O, HF, LiF, N2, CO, NO, O2, and F2, two very promising new model chemistries emerge. The first is of comparable accuracy, but more than ten times the speed of the G1 model of Pople and co‐workers. The second is less than one‐tenth the speed of the G1 model, but reduces the root‐mean‐square (rms) errors in ionization potentials (IPs), electron affinities (EAs), and D0’s to 0.033 and 0.013 eV, and 0.53 kcal/mol per bond, respectively.

Pseudopotential approaches to Ca, Sr, and Ba hydrides. Why are some alkaline earth MX2 compounds bent?

Abstract: Quasirelativistic and nonrelativistic 10‐valence‐electron pseudopotentials for Ca, Sr, and Ba are presented. Results of calculations with 6s6p5d basis sets for MH, MH+, and MH2 are compared with all‐electron and 2‐valence‐electron pseudopotential calculations with and without core‐polarization potentials. The 10‐valence‐electron pseudopotential approach agrees well with all‐electron calculations. It circumvents problems for the 2‐valence‐electron pseudopotentials arising from an incomplete separation of valence and subvalence shells in polar molecular systems due to strongly contracted occupied (n−1)‐d orbitals. All higher‐level calculations show SrH2 and BaH2 to be bent with angles of ∼140° and 120°, respectively, while CaH2 is linear with a flat potential‐energy surface for the bending motion. The use of a core‐polarization potential together with the 2‐valence‐electron pseudopotential approach allows an investigation of the relative importance of core‐polarization vs direct d‐orbital bonding participation as reasons for the bent structures. The calculations strongly suggest that both contribute to the bending in SrH2 and BaH2. Even at the Hartree–Fock level of theory 10‐valence‐electron pseudopotential calculations given reasonable angles when the potential‐energy surface is not exceedingly flat, and only moderately contracted basis sets including both compact d functions and diffuse p functions are used. The effect of core‐valence correlation and the importance of f functions also are discussed.

Interdiffusion and self‐diffusion in polymer mixtures: A Monte Carlo study

Abstract: A lattice model for dense polymer solutions and polymer mixtures in three dimensions is presented, aiming to develop a model suitable for efficient computer simulation on vector processors, with a qualitatively realistic local dynamics. It is shown that the bond fluctuation algorithm for a suitable set of allowed bond vectors has the property that due to the excluded volume constraint no crossing of bonds by local motions can occur, and entanglement restrictions thus are fully taken into account. For athermal binary (AB) symmetrical polymer mixtures, the dependence of both self‐diffusion coefficient and interdiffusion coefficient on polymer density is obtained, simulating a thin film geometry where a film of polymer A is coated with a film of polymer B. For one density, the dependence of the interdiffusion coefficient on an attractive energy between unlike monomers is also studied. For weak attraction an enhancement of interdiffusion proportional to this energy occurs. For strong attraction, however, a rather immobile tightly bound AB layer forms in the interface which hampers further unmixing.

Extension of Gaussian‐1 (G1) theory to bromine‐containing molecules

Abstract: Bromine bases suitable for computing G1 energies of bromine‐containing molecules have been derived. Our recommended procedure for calculating such energies adopts the G1 steps previously introduced by Pople and co‐workers with the following modifications: (i) second‐order Mo/ller–Plesset (MP2) geometry optimizations use the polarized, split‐valence SV4P basis set for bromine along with 6‐31G(d) for first‐ and second‐row atoms; (ii) fourth‐order Mo/ller–Plesset (MP4) and QCISD(T) energies are calculated with our new bromine bases along with supplemented 6‐311G and McLean–Chandler (MC) 6‐311G bases for first‐ and second‐row atoms; and (iii) bromine atomic spin–orbit corrections are explicitly taken into account. G1 energies have been calculated for a selection of simple processes involving bromine‐containing molecules. The results obtained are within 0.1 eV of experiment except for the ionization energy of Br2, where the inclusion of molecular spin–orbit corrections is necessary to achieve 0.1 eV accuracy.

Fluorescence-detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses

Abstract: We introduce a novel spectroscopic technique which utilizes a two‐pulse sequence of femtosecond duration phase‐locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in‐phase, in‐quadrature, and out‐of‐phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct de...

Structures of carbon cluster ions from 3 to 60 atoms: Linears to rings to fullerenes

Abstract: A new method for identifying and characterizing cluster ion isomers is presented Results indicate that most carbon clusters have more than one stable form, with 29≤n≤45 indicating three or four different structures Fullerenes first appear at C+30 and begin to dominate above C+45 From small to large the isomeric progression is from linear to rings to fullerenes

Analytic energy derivatives in the numerical local‐density‐functional approach

Abstract: Analytical energy gradients for numerical orbital expansions can be calculated using the same three‐dimensional integration methods as for calculating the total energy in the local‐density‐functional approach. It is shown that in addition to Pulay corrections for expansion functions attached to the atomic sites correction terms for non‐self‐consistency of the auxiliary density can also be used with benefit. The usefulness of this approach is demonstrated in the calculation of equilibrium geometries of organic and inorganic molecules, radicals, and transition‐metal compounds. The calculated structural parameters are in at least as good agreement with experimental data as structures obtained from standard ab initio methods. Excellent basis sets can be used at a comparably low computational cost.

Femtosecond solvation dynamics in acetonitrile: Observation of the inertial contribution to the solvent response

Abstract: The solvation dynamics of acetonitrile were characterized by a time resolved fluorescence shift measurement determined via the fluorescence upconversion technique. The solvation response is clearly two part in character. The fast initial relaxation accounts for ∼80% of the amplitude and is well fit by a Gaussian of 120 fs FWHM, giving a decay time of 70 fs. The slower tail is exponential with a decay time of ∼200 fs. Comparison of the results to molecular dynamics simulations performed by Maroncelli [J. Chem. Phys. 94, 2085 (1991)] reveal the fast initial part of the solvent response arises from small amplitude inertial rotational motion of molecules in the first solvation shell. The implications of a large amplitude, rapid inertial Gaussian component in the solvent response for theoretical descriptions of chemical reaction dynamics in solution are discussed.

Potential energy surfaces near intersections

Abstract: The topographies of two potential energy surfaces are examined in the vicinity of their intersection. A brief account of the basic theory is given and the possible surface types are discussed explicitly. Two main patterns are found. One of these (‘‘peaked’’) has the character of a tilted double cone in that the lower (upper) surface decreases (increases) in all directions from the intersection which is a point where an infinite number, in fact, all orthogonal trajectories emanate. The other pattern (‘‘sloped’’) results when both surfaces are monotonically sloped and touch each other along the slope, with most orthogonal trajectories bypassing the intersection. When the latter pattern prevails, the intersection can lie on a steepest descent line which originates at a transition state and hence may qualify as a reaction path model. An intermediate pattern, involving a horizontal slope on both surfaces, is also possible. The topographical patterns also differ markedly with respect to the bunching of the steepest descent lines. In general, the latter tend to veer away from the intersection on the lower surface favoring bifurcations, but are funneled towards the intersection on the upper surface, making the vicinity of the intersection a region favoring radiationless transitions. The various cases are classified and illustrated through quantitative graphs of contours and orthogonal trajectories.

Computer simulations of solvation dynamics in acetonitrile

Abstract: Computer simulations of the solvation of monatomic ions in acetonitrile are used to investigate dynamical aspects of solvation in polar aprotic solvents. The observed dynamics depend significantly on solute charge and on which multipole moment of the solute is perturbed. In all cases, the solvation response has a two‐part character. One part consists of a fast initial relaxation and attendant oscillations, both of which occur on a time scale of 0.1–0.2 ps. The initial response is well fit by a Gaussian function and accounts for ∼80% of the total relaxation. The second dynamical component occurs on a much slower, ∼1 ps time scale, and accounts for the remainder of the relaxation. The fast response results from small amplitude inertial dynamics of solvent molecules within the confines of their instantaneous environment. The slow component reflects larger amplitude motions involving the breakup and reorganization of these local environments, especially in the first solvation shell of the solute. Comparison o...

Dynamics and rheology of complex interfaces. I

Abstract: When a concentrated mixture of two immiscible fluid is sheared, a rather complex interface is formed due to the coagulation, rupture, and deformation of droplets. The dynamics and rheological properties of such system is discussed. A semiphenomenological kinetic equation is proposed which describes the time evolution of droplet size and orientation, and also the macroscopic stress in a flow field. The rheological properties are shown to be quite unusual: for example, the steady‐state viscosity is independent of the shear rate, while the normal stress difference is nonzero and proportional to the magnitude of the shear rate.

Solvation dynamics for an ion pair in a polar solvent: Time‐dependent fluorescence and photochemical charge transfer

Abstract: The results of a molecular dynamics (MD) computer simulation are presented for the solvation dynamics of an ion pair instanteously produced from a neutral pair, in a model polar aprotic solvent. These time‐dependent fluorescence dynamics are analyzed theoretically to examine the validity of several linear response theory approaches, as well as of various theoretical descriptions (e.g., Langevin equation) for the solvent dynamics per se. It is found that these dynamics are dominated for short times by a simple inertial Gaussian behavior, a feature which is absent in many current theoretical treatments, and which is related to the approximate validity of linear response theory. Nonlinear aspects, such as an overall spectral narrowing, but a transient initial spectral broadening, are also discussed. A model photochemical charge transfer process is also briefly considered to elucidate aspects of the connection between solvation dynamics and chemical kinetic population evolution.

Mass analyzed threshold ionization spectroscopy

Abstract: The developement of a new mass selective method is reported for the determination of the optical spectra of molecular ions and for the production of state selected ions.(AIP)

Charge transport in disordered molecular solids

Abstract: Hole mobilities have been measured in vapor deposited films of 1,1‐bis(di‐4‐tolylaminophenyl)cyclohexane (TAPC) and TAPC‐doped bisphenol‐A‐polycarbonate (BPPC). Over an extended range of temperatures, the mobilities decrease with increasing field at low fields. At high fields, a log μ∝E1/2 relationship is observed with a slope that approaches zero at high temperatures. The results are described within the framework of the disorder transport formalism. By comparison of the experimental results with Monte Carlo simulations, we show that the observed behavior is a signature of the simultaneous presence of diagonal and off‐diagonal disorder. Agreement between simulation results and experiment is excellent. Generalizing these results provides a framework for determining the magnitude of the relevant diagonal and off‐diagonal disorder parameters from an analysis of mobility measurements.

An examination of ab initio results for the helium potential energy curve

Abstract: Obtaining a ground state potential energy curve for helium has been the subject of much research involving empirical, semiempirical, and ab initio methods. In this work, we examine critically recent ab initio potentials proposed for this interaction with respect to their ability to predict certain accurate experimental data. To accomplish this analysis, potentials with a modified HFD‐B form were fit to the recent theoretical work of van Duijneveldt and co‐workers [Vos, van Lenthe, and van Duijneveldt, J. Chem. Phys. 93, 643 (1990) and Vos, van Mourik, van Lenthe, and van Duijneveldt (to be published)] and Liu and McLean (LM‐2) [J. Chem. Phys. 91, 2348 (1989)]. A well depth (e/k=10.92 K) and a separation at the minimum (rm=2.9702 A) consistent with both determinations were chosen and the properties of helium were calculated based on these potentials. These ‘‘mimic’’ potentials fail to predict the very low temperature 4He and 3He virials and one of them [Vos, van Maurik, van Lenthe, and van Duijneveldt (to ...

Hartree–Fock second derivatives and electric field properties in a solvent reaction field: Theory and application

Abstract: A compact formalism for the second and third derivatives of the Hartree–Fock energy in the presence of an Onsager solvent reaction field is presented. All three standard algorithms (MO, AO, and direct) are extended to include the reaction field in a unified way. Predictions of the infrared spectrum of formaldehyde in a variety of solvents and of solvent‐induced shifts in carbonyl stretching frequencies are presented along with the results of new measurements. As for the gas‐phase case, analytical second derivatives are far more efficient than numerical ones. The reaction field provides very good predictions of solvent effects at negligible computational cost.

Cellular dynamics: A new semiclassical approach to time‐dependent quantum mechanics

Abstract: A new semiclassical approach that constructs the full semiclassical Green’s function propagation of any initial wave function directly from an ensemble of real trajectories, without root searching, is presented. Each trajectory controls a cell of initial conditions in phase space, but the cell area is not constrained by Planck’s constant. The method is shown to be accurate for rather long times in anharmonic oscillators, indicating the semiclassical time‐dependent Green’s function is clearly worthy of more study. The evolution of wave functions in anharmonic potentials is examined and a spectrum from the semiclassical correlation function is calculated, comparing with exact fast Fourier transform results.

The energetics and structure of nickel clusters: Size dependence

Abstract: The energetics of nickel clusters over a broad size range are explored within the context of the many‐body potentials obtained via the embedded atom method. Unconstrained local minimum energy configurations are found for single crystal clusters consisting of various truncations of the cube or octahedron, with and without (110) faces, as well as some monotwinnings of these. We also examine multitwinned structures such as icosahedra and various truncations of the decahedron, such as those of Ino and Marks. These clusters range in size from 142 to over 5000 atoms. As in most such previous studies, such as those on Lennard‐Jones systems, we find that icosahedral clusters are favored for the smallest cluster sizes and that Marks’ decahedra are favored for intermediate sizes (all our atomic systems larger than about 2300 atoms). Of course very large clusters will be single crystal face‐centered‐cubic (fcc) polyhedra: the onset of optimally stable single‐crystal nickel clusters is estimated to occur at 17 000 atoms. We find, via comparisons to results obtained via atomistic calculations, that simple macroscopic expressions using accurate surface, strain, and twinning energies can usefully predict energy differences between different structures even for clusters of much smaller size than expected. These expressions can be used to assess the relative energetic merits of various structural motifs and their dependence on cluster size.

Theory of polyampholyte solutions

Abstract: We consider polyampholyte polymers containing both positive and negative monomers randomly dispersed along the chain. Neutral chains collapse into a globule due to attractive electrostatic interactions. The behavior of the charges inside the globule is similar to that of charges in a small volume of simple electrolyte. A screening length κ−1p coming from the polymeric charge may be defined as in Debye–Huckel theory. The internal structure of the globule is that of close packed blobs of radius equal to the screening length. When salt is added this further screens the interactions and reduces the attractions. The globule begins to increase in size when the concentration of salt becomes larger than the concentration of charge on the polymer itself. Screened Coulomb interactions in a neutral chain behave like a negative contribution to excluded volume. For a chain in a good solvent there is a θ salt concentration at which the net excluded volume becomes zero. Chains are swollen above this concentration of salt, and collapsed below this concentration. For small sections of chain the Coulomb interactions are unscreened and cannot be treated as a modification to excluded volume. Chains with a strong net charge of one sign tend to behave as conventional polyelectrolyte with charges of only one sign. We determine the criterion for the value of the net charge at which the repulsions (polyelectrolyte effect) begin to dominate the attractions (polyampholyte effect). The predictions are found to be in good qualitative agreement with experiments.

Detection and spectroscopy of single pentacene molecules in a p‐terphenyl crystal by means of fluorescence excitation

Abstract: Recent advances in fluorescence excitation spectroscopy with high efficiency have produced greatly improved optical spectra for the first electronic transition of individual single molecules of pentacene in p‐terphenyl crystals at low temperatures (1.5 to 10 K). Two classes of single molecule behavior are observed: class I molecules have time‐independent resonance frequencies, and class II molecules show a diffusive motion among several resonant frequencies with time which we term ‘‘spectral diffusion’’ by analogy with a similar effect which is common in amorphous materials. The temperature dependence of the linewidth and the power dependence of the fluorescence emission rate and of the linewidth are reported and analyzed. Various forms of the surprising class II behavior are described, including jumping among several discrete frequencies, creeping toward the center of the inhomogeneous line in many small steps, and a wandering among many possible resonance frequencies. The occurrence of class II behavior is restricted to the wings of the inhomogeneous line suggesting that the effect is correlated with some form of local disorder. The spectral diffusion rate increases with increasing temperature, suggesting that the effect may be due to phonon‐assisted transitions of local degrees of freedom around the pentacene defect whose source remains to be identified conclusively.

Reactions of boron atoms with molecular oxygen. Infrared spectra of BO, BO2, B2O2, B2O3, and BO−2 in solid argon

Abstract: Boron atoms from Nd:YAG laserablation of the solid have been codeposited with Ar/O2 samples on a 11±1 K salt window. The product infrared spectrum was dominated by three strong 11B isotopic bands at 1299.3, 1282.8, and 1274.6 cm− 1 with 10B counterparts at 1347.6, 1330.7, and 1322.2 cm− 1. Oxygen isotopic substitution (16O18O and 18O2 ) confirms the assignment of these strong bands to ν3 of linear BO2. Renner–Teller coupling is evident in the ν2 bending motion. A sharp medium intensity band at 1854.7 has appropriate isotopic ratios for BO, which exhibits a 1862.1 cm− 1 gas phase fundamental. A sharp 1931.0 cm− 1 band shows isotopic ratios appropriate for another linear BO2 species; correlation with spectra of BO− 2 in alkali halide lattices confirms this assignment. A weak 1898.9 cm− 1 band grows on annealing and shows isotopic ratios for a BO stretching mode and isotopic splittings for two equivalent B and O atoms, which confirms assignment to B2O2. A weak 2062 cm− 1 band grows markedly on annealing and shows isotope shifts appropriate for a terminal–BO group interacting with another oxygen atom; the 2062 cm− 1 band is assigned to B2O3 in agreement with earlier work. A strong 1512.3 cm− 1 band appeared on annealing; its proximity to the O2 fundamental at 1552 cm− 1 and pure oxygen isotopic shift suggest that this absorption is due to a B atom–O2 complex.

An electronic Hamiltonian for origin independent calculations of magnetic properties

Abstract: A gauge origin independent formalism for the calculation of molecular magnetic properties is presented. Origin independence is obtained by using London’s gauge invariant atomic orbitals, expanding the second quantization Hamiltonian in the external magnetic field and nuclear magnetic moments, and using the resulting expansion terms as perturbation operators in response function calculations. To ensure orthonormality of the molecular orbitals, a field‐dependent symmetrical orthonormalization is employed. In this way the gauge dependence of the London orbitals is transferred to the Hamiltonian. The resulting perturbation operators may be used to calculate magnetic properties from any approximate ab initio wave function.

Many-body effects in molecular dynamics simulations of Na +(H2O)n and Cl-(H2O) n clusters

Abstract: Many‐body effects were examined in a series of molecular dynamics computer simulations on the ionic aqueous clusters Na+(H2O)n (n=4,5,6,14) and Cl−(H2O)n (n=4,5,6,7,8,14). Two potential models were used in the simulations. In one model (TIP4P) the potential was pairwise additive, while in the second model (SPCE/POL) the many body effects were explicitly included through a self‐consistent polarization routine. The two models produce equilibrium structures which are significantly different in energy and geometry. The SPCE/POL model consistently predicts energetically more stable products. In addition, for the anion cluster systems the SPCE/POL model places the Cl− on the surface of the water cluster.

Generalized correlations in terms of polarizability for van der Waals interaction potential parameter calculations

Abstract: General correlations between van der Waals interaction potential parameters and polarizabilities of the interacting neutral–neutral partners of any nature are presented and discussed. To ensure the full applicability of the correlations, an evaluation of the long‐range interaction constants is performed in terms of the Slater–Kirkwood approximation whose numerical coefficients, having the meaning of effective electron numbers, are estimated interpolating the values deduced by theoretical considerations. The values of the long‐range constants so obtained are compared satisfactorily with the available experimental ones. The correlations are tested successfully over practically all systems characterized experimentally. Their use to predict the parameters of unknown systems is suggested.