Showing papers on "Ab initio quantum chemistry methods published in 1997"

On the quantum nature of the shared proton in hydrogen bonds

Abstract: The relative influence of thermal and quantum fluctuations on the proton transfer properties of the charged water complexes H5O2+ and H3O2− was investigated with the use of ab initio techniques. These small systems can be considered as prototypical representatives of strong and intermediate-strength hydrogen bonds. The shared proton in the strongly hydrogen bonded H5O2+ behaved in an essentially classical manner, whereas in the H3O2− low-barrier hydrogen bond, quantum zero-point motion played a crucial role even at room temperature. This behavior can be traced back to a small difference in the oxygen-oxygen separation and hence to the strength of the hydrogen bond.

Opls all-atom force field for carbohydrates

Abstract: The OPLS all-atom (AA) force field for organic and biomolecular systems has been expanded to include carbohydrates. Starting with reported nonbonded parameters of alcohols, ethers, and diols, torsional parameters were fit to reproduce results from ab initio calculations on the hexopyranoses, α,β-d-glucopyranose, α,β-d-mannopyranose, α,β-d-galactopyranose, methyl α,β-d-glucopyranoside, and methyl α,β-d-mannopyranoside. In all, geometry optimizations were carried out for 144 conformers at the restricted Hartree–Fock (RHF)/6–31G* level. For the conformers with a relative energy within 3 kcal/mol of the global minima, the effects of electron correlation and basis-set extension were considered by performing single-point calculations with density functional theory at the B3LYP/6–311+G** level. The torsional parameters for the OPLS-AA force field were parameterized to reproduce the energies and structures of these 44 conformers. The resultant force field reproduces the ab initio calculated energies with an average unsigned error of 0.41 kcal/mol. The α/β ratios as well as the relative energies between the isomeric hexopyranoses are in good accord with the ab initio results. The predictive abilities of the force field were also tested against RHF/6–31G* results for d-allopyranose with excellent success; a surprising discovery is that the lowest energy conformer of d-allopyranose is a β anomer. © 1997 John Wiley & Sons, Inc. J Comput Chem18: 1955–1970, 1997

Tight-binding modelling of materials

Abstract: The tight-binding method of modelling materials lies between the very accurate, very expensive, ab initio methods and the fast but limited empirical methods. When compared with ab initio methods, tight-binding is typically two to three orders of magnitude faster, but suffers from a reduction in transferability due to the approximations made; when compared with empirical methods, tight-binding is two to three orders of magnitude slower, but the quantum mechanical nature of bonding is retained, ensuring that the angular nature of bonding is correctly described far from equilibrium structures. Tight-binding is therefore useful for the large number of situations in which quantum mechanical effects are significant, but the system size makes ab initio calculations impractical. In this paper we review the theoretical basis of the tight-binding method, and the range of approaches used to exactly or approximately solve the tight-binding equations. We then consider a representative selection of the huge number of systems which have been studied using tight-binding, identifying the physical characteristics that favour a particular tight-binding method, with examples drawn from metallic, semiconducting and ionic systems. Looking beyond standard tight-binding methods we then review the work which has been done to improve the accuracy and transferability of tight-binding, and moving in the opposite direction we consider the relationship between tight-binding and empirical models.

Theory of the d10–d10 Closed‐Shell Attraction: 1. Dimers Near Equilibrium

Abstract: We study the dependence of the aurophilic attraction (AuI–AuI) in perpendicular model systems of the type [(ClAuPH3)2] on the ab initio method, basis set and different pseudopotentials used, and on relativity. The effects of varying the „halogen” (X = F, H, Cl, Me, Br, -C≡CH, I), the „phosphine” (L = PH3, PMe3, -N≡CH) and the metal (M = Cu, Ag, Au) on the M–M′ interaction of the [(XML)2] dimer are also studied. The depth of the interaction potential increases with the softness of the group X. It decreases by 27% for M = Au, X = Cl and L = PH3 if relativistic effects are omitted at fixed geometry.

Accurate ab Initio Quantum Chemical Determination of the Relative Energetics of Peptide Conformations and Assessment of Empirical Force Fields

Abstract: Correlated ab initio calculations have been carried out with a parallel version of the PSGVB electronic structure code to obtain relative energetics of a number of conformations of the alanine tetrapeptide. The highest level of theory utilized, local MP2 with the cc-pVTZ(−f) correlation-consistent basis set, has previously been shown to provide accurate conformational energies in comparison with experiment for a data set of small molecules. Comparisons with published and new canonical MP2 calculations on the alanine dipeptide are made. Results for ten gas-phase tetrapeptide conformations and a β-sheet dipeptide dimer are compared with 20 different molecular mechanics force field parametrizations, providing the first assessment of the reliability of these models for systems larger than a dipeptide. Comparisons are made with the LMP2/cc-pVTZ(−f) results, which are taken as a benchmark for the tetrapeptides. Statistical summaries with regard to energetics and structure are produced for each force field, and ...

The prediction of molecular equilibrium structures by the standard electronic wave functions

Abstract: A systematic investigation has been carried out of the accuracy of molecular equilibrium structures of 19 small closed-shell molecules containing first-row atoms as predicted by the following standard electronic ab initio models: Hartree–Fock (HF) theory, Mo/ller–Plesset theory to second, third, and fourth orders (MP2, MP3, and MP4), coupled-cluster singles and doubles (CCSD) theory; CCSD theory with perturbational triples corrections [CCSD(T)], and the configuration-interaction singles and doubles (CISD) model. For all models, calculations were carried out using the correlation-consistent polarized valence double-zeta (cc-pVDZ) basis, the correlation-consistent polarized valence triple-zeta (cc-pVTZ) basis, and the correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis. Improvements in the basis sets shorten the bond distances at all levels. Going from cc-pVDZ to cc-pVTZ, bond distances are on the average reduced by 0.8 pm at the Hartree–Fock level and by 1.6 pm at the correlated levels. From cc-pVTZ to cc-pVQZ, the contractions are about ten times smaller and the cc-pVTZ basis set appears to yield bond distances close to the basis-set limit for all models. The models HF, MP2, and CCSD(T) give improved accuracy at increased computational cost. The accuracy of the Mo/ller–Plesset series oscillates, with MP3 being considerably less accurate than MP2 and MP4. The MP2 geometries are remarkably accurate, being only very slightly improved upon at the MP4 level for the cc-pVQZ basis. The CCSD equilibrium structures are only moderately accurate, being intermediate between MP2 and MP3. The accuracy of the CCSD(T) model, in contrast, is high and comparable to that observed in most experimental studies and it has been used to challenge the experimentally determined equilibrium structure of HNO. The CISD wave function provides structures of low quality.

Langevin Dipoles Model for ab Initio Calculations of Chemical Processes in Solution: Parametrization and Application to Hydration Free Energies of Neutral and Ionic Solutes and Conformational Analysis in Aqueous Solution

Abstract: A new parametrization of the Langevin dipole (LD) model is developed for ab initio calculations of chemical processes in aqueous solution. This parametrization is implemented in both the iterative (ILD) and noniterative (NLD) versions of the LD model. The training set for the new parametrization encompassed solvation free energies of 44 neutral and 39 ionic solutes that contained the C, O, N, P, S, F, and Cl atoms. The performance of the model is assessed by examining its ability to represent the overall training set, pKa differences of structurally related compounds, and conformation-related changes in the solvation energy of 1,2-ethanediol (glycol). The effects of solute polarization and electron correlation are also discussed. The overall performance of the model is found to be comparable or slightly better than the PCM continuum model of Tomasi and co-workers. However, the simplified explicit representation of solvent molecules of the LD model may allow one to gain a somewhat clearer insight into the ...

Performance of empirical potentials (AMBER, CFF95, CVFF, CHARMM, OPLS, POLTEV), semiempirical quantum chemical methods (AM1, MNDO/M, PM3), and ab initio Hartree–Fock method for interaction of DNA bases: Comparison with nonempirical beyond Hartree–Fock results

Abstract: Empirical energy functions (AMBER 4.1, CFF95, CHARMM23, OPLS, Poltev), semiempirical quantum chemical methods (AM1, MNDO/M, PM3), and the nonempirical ab initio self-consistent field (SCF) method utilizing a minimal basis set combined with the London dispersion energy (SCFD method) were used for calculation of stabilization energies of 26 H-bonded DNA base pairs, 10 stacked DNA base pairs (thymine was replaced by uracil), and the B-DNA decamer (only DNA bases were considered). These energies were compared with nonempirical ab initio beyond Hartree–Fock values [second-order Moller–Plesset (MP2)/6–31G*(0.25)]. The best performance was exhibited by AMBER 4.1 with the force field of Cornell et al. The SCFD method, tested for H-bonded pairs only, exhibited stabilization energies that were too large. Semiempirical quantum chemical methods gave poor agreement with MP2 values in the H-bonded systems and failed completely for stacked pairs. A similar failure was recently reported for density functional theory calculations on base stacking. It may be concluded that currently available force fields provide much better descriptions of interactions of nucleic acid bases than the semiempirical methods and low-level ab initio treatment. © 1997 John Wiley & Sons, Inc. J Comput Chem18: 1136–1150

Natural bond orbital analysis of steric interactions

Abstract: We describe an ab initio procedure for extracting the Pauli exchange antisymmetry (“steric”) contributions to molecular potential energy in the framework of self-consistent-field molecular orbital (SCFMO) theory. This “natural steric analysis” method is based on natural bond orbital (NBO) representation of the SCFMO wave function, which allows the steric exchange energy to be approximated as an energy difference between “preorthogonal” and final NBOs, analogous to the procedure of Sovers et al. [J. Chem. Phys. 49, 2592 (1965)]. We show how the total NBO steric exchange energy can in turn be approximated in terms of pairwise-additive interactions between localized bonding units, comparable to the empirical steric potentials of molecular mechanics models. The accuracy of NBO steric analysis is tested in applications to various rare-gas interactions (expected to be of pure steric exchange type), and excellent agreement is found with the full ab initio potential curves over a wide range of separations. Limitations of the simple pairwise-additive model of steric exchange interactions are noted and characterized.

Interaction of DNA Base Pairs with Various Metal Cations (Mg2+, Ca2+, Sr2+, Ba2+, Cu+, Ag+, Au+, Zn2+, Cd2+, and Hg2+): Nonempirical ab Initio Calculations on Structures, Energies, and Nonadditivity of the Interaction

Abstract: Interaction of Watson−Crick adenine−thymine (AT) and guanine−cytosine (GC) base pairs with various metal (M) cations (Mg2+, ..., Hg2+) were studied by nonempirical ab initio methods with inclusion of correlation energy. Cations were allowed to interact with the N7 nitrogen of adenine and the N7 and O6 atoms of guanine. All of the cations were described by Christiansen's average relativistic effective potentials using the DZ+P basis set, while the 6-31G** basis set was used for the elements of base pairs. Disruption of the adenine−thymine as well as guanine−cytosine pairs in the presence of all studied cations is energetically more demanding than that for isolated base pairs; the addition stabilization of the base pair is about 100% for complexes with dication. The interaction is highly nonadditive. The three-body term is for the MGC complex considerably larger than that for MAT; the intercomplex charge transfer is also much larger for the former complex.

Contribution of Many-Body Terms to the Energy for Small Water Clusters: A Comparison of ab Initio Calculations and Accurate Model Potentials

Abstract: We have tested the ability of two new model potentials constructed using intermolecular perturbation theory methods to reproduce ab initio results at a comparable level of theory. Several configurations of water trimer, tetramer, and pentamer are studied, and in addition to the contributions to the interaction energy, the potential energy surfaces are compared by optimizing the model potential geometries to local stationary points within a rigid-body framework. In general the agreement between the two methods is good, validating the model potentials as suitable candidates for providing starting geometries for further ab initio calculations and for the simulation of larger systems.

Exchange and correlation energy in density functional theory. Comparison of accurate DFT quantities with traditional Hartree-Fock based ones and generalized gradient approximations for the molecules Li2, N2, F2.

Abstract: The density functional definition of exchange and correlation differs from the traditional one. In order to calculate the density functional theory (DFT), quantities accurately, molecular Kohn–Sham (KS) solutions have been obtained from ab initio wave functions for the homonuclear diatomic molecules Li2, N2, F2. These afford the construction of the KS determinant Ψs and the calculation of its total electronic energy EKS and the kinetic, nuclear-attraction and Coulomb repulsion components Ts, V, WH as well as the (DFT) exchange energy Ex and correlation energy Ec. Comparison of these DFT quantities has been made on one hand with the corresponding Hartree–Fock (HF) quantities and on the other hand with local density approximation (LDA) and generalized gradient approximation (GGA). Comparison with HF shows that the correlation errors in the components T, V, and WH of the total energy are much larger for HF than KS determinantal wave functions. However, the total energies EKS and EHF appear to be close to eac...

An accurate potential energy curve for helium based on ab initio calculations

Abstract: Korona, Williams, Bukowski, Jeziorski, and Szalewicz [J. Chem. Phys. 106, 1 (1997)] constructed a completely ab initio potential for He2 by fitting their calculations using infinite order symmetry adapted perturbation theory at intermediate range, existing Green’s function Monte Carlo calculations at short range and accurate dispersion coefficients at long range to a modified Tang–Toennies potential form. The potential with retardation added to the dipole-dipole dispersion is found to predict accurately a large set of microscopic and macroscopic experimental data. The potential with a significantly larger well depth than other recent potentials is judged to be the most accurate characterization of the helium interaction yet proposed.

Hydration and Water Exchange of Zinc(II) Ions. Application of Density Functional Theory

Abstract: Details of the hydration and water exchange mechanism of Zn2+ have been studied using density functional calculations with a variety of different basis sets. The computed structures and hydration energies for complexes of the type [Zn(H2O)n]2+ with n = 1−6 are in good agreement with previous results obtained from ab initio calculations and self-consistent reaction field methods. Extension of our investigations to the second coordination (first solvation) sphere and thus to complexes of the general type [Zn(H2O)n]2+·mH2O with n = 5 and m = 1, 2 and n = 6 and m = 1 reveals two types of complexes having either one or two hydrogen bonds between first and second sphere water molecules. The water exchange mechanism of [Zn(H2O)6]2+ is analyzed on the basis of the structures and energies of these complexes. Within the variations due to the different basis sets employed, the Zn−O bond length for water molecules in the first coordination sphere is between 2.0 and 2.1 A, water molecules in the second coordination sp...

Six dimensional quantum dynamics study for dissociative adsorption of H2 on Cu(111) surface

Abstract: In this letter we present preliminary results of full six dimensional quantum dynamics calculations for dissociative adsorption of a hydrogen molecule on a Cu(111) surface. We utilize the time-dependent wave-packet approach to simulate the dissociation process on a full dimensional LEPS potential energy surface which has incorporated the latest ab initio data [Hammer et al. Phys. Rev. Lett. 73, 1400 (1994)]. We use a novel partitioning of the angular momentum operator in the split-operator method so that a direct product DVR can be rigorously implemented. The most interesting observation in the present rigorous quantum dynamics study is the site-averaged effect, i.e., the averaged dissociation probability of the four dimensional calculations over the three symmetric impact sites strongly resembles the exact dissociation probability of the six dimensional calculations. In accord with the low dimensional calculations, initial vibrational excitation of H2 effectively reduces the translational threshold energ...

Practical methods in ab initio lattice dynamics

Abstract: A popular method of extracting phonon frequencies from ab initio calculations is to find the equilibrium structure of a material and then build up the matrix of force constants by calculating forces acting due to small displacements of the atoms. If the range of the force constants is assumed to be short, as it typically is in parametrized force-model calculations, the entire dispersion relation can be evaluated from data taken from small ab initio calculations. In this paper we highlight the practical difficulties introduced by low-symmetry structures with free internal parameters and present practical solutions to them. By way of example, we present a number of calculations where solution of these problems is essential. These include ab initio calculation of phonon dispersion in a non-centrosymmetric structure (silver gallium diselenide) and good agreement between calculations and neutron scattering data for a structure with free internal parameters (germanium sulphide).

The torsional conformations of butane: Definitive energetics from ab initio methods

Abstract: The torsional potential function for butane was probed theoretically using increasingly complete basis sets (up to 840 functions) and treatments of electron correlation [up to the CCSD(T) method] until it was shown that the sequence of relative energies approached convergence. The Schrodinger limit in the Born–Oppenheimer approximation was thus estimated. The equilibrium energies relative to the anti conformation (ΔEe) as obtained by this focal-point extrapolation were 0.62, 3.31, and 5.51 kcal mol−1 for the geometrically optimized stationary points having carbon backbone torsion angles of 64.8°, 119.6°, and 0°, respectively. The final prediction of the anti–syn difference is 5.40±0.15 kcal mol−1. Some consequences of this result are discussed.

Absolute Configuration of Bromochlorofluoromethane from Experimental and Ab Initio Theoretical Vibrational Raman Optical Activity

Abstract: After more than 100 years, the absolute configurations of the dextro- and levorotatory enantiomers of bromochlorofluoromethane 1, one of the simplest chiral molecules, have now been determined. Comparison of the measured Raman optical activity spectrum (in the range of vibrational modes) of an enantiomerically enriched sample of 1 with the calculated spectra allow the assignments (S)-(+) and R-(−) to be made.

Absolute Affinities of α-Amino Acids for Cu+ in the Gas Phase. A Theoretical Study

Abstract: Ab initio calculations have been carried out on the [glycine−Cu]+, [serine−Cu]+, and [cysteine−Cu]+ complexes. Investigation of several types of structures for each complex shows that the preferred binding site of Cu+ involves chelation between the carbonyl oxygen and the amino nitrogen. With glycine, this leads to a complexation energy (best estimate of D0) of 64.3 kcal/mol. Additional chelation with the alcohol group of serine or the thiol group of cysteine leads to larger binding energies, with cysteine binding more strongly than serine, in good agreement with a recent experimental scale of relative Cu+ affinities of all α-amino acids present in natural peptides. Combining this scale to the accurate determination of the Cu+ affinity of glycine from the present work leads to absolute values of Cu+ affinities of all amino acids. Calculations were also carried out on the complexes of Cu+ with water, ammonia, formaldehyde, and hydrogen sulfide. The geometrical and electronic structures of these complexes a...

A study of the structure and bonding of small aluminum oxide clusters by photoelectron spectroscopy: AlxOy− (x=1–2, y=1–5)

Abstract: The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and B 2Σg+ excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermore, the ground state vibrational frequencies of these three molecules are also similar. These observations lead us to suggest that these molecules all have a rhombuslike structure, similar to Al2O2, with the oxygen atoms sequentially attaching to the terminal aluminum atoms. The spectra are consistent with an ionic bonding view of these clusters and the vibrational frequencies are in good agreement with the theoretical results. Significant information about the structure and bonding of these small aluminum oxide clusters is obtained and discussed.

Lithium–aluminum contacts for organic light-emitting devices

Abstract: Organic light-emitting devices have been prepared with multilayer Al–Li–Al cathodes in an ultrahigh vacuum molecular beam deposition system The optimum device characteristics are obtained when there is a single Al layer separating the Li layer from the organic materials Ab initio molecular dynamics calculations of the Al–Li interaction clarify the role of Al as a blocking layer to Li diffusion into the organic films as well as the behavior of the device when the thickness of this Al interfacial layer is changed

Yielding and fracture mechanisms of nanowires

Abstract: This paper presents a detailed analysis of atomic structure and force variations in metal nanowires under tensile strain. Our work is based on state of the art molecular dynamics simulations and ab initio self-consistent field calculations within the local density approximation, and predicts structural transformations. It is found that yielding and fracture mechanisms depend on the size, atomic arrangement, and temperature. The elongation under uniaxial stress is realized by consecutive quasielastic and yielding stages; the neck develops by the migration of atoms, but mainly by the sequential implementation of a new layer with a smaller cross section at certain ranges of uniaxial strain. This causes an abrupt decrease of the tensile force. Owing to the excessive strain at the neck, the original structure and atomic registry are modified; atoms show a tendency to rearrange in closed-packed structures. In certain circumstances, a bundle of atomic chains or a single atomic chain forms as a result of transition from the hollow site to the top site registry shortly before the break. The wire is represented by a linear combination of atomic pseudopotentials and the current is calculated to investigate the correlation between conductance variations and atomic rearrangements of the wire during the stretch. The origin of the observed ``giant'' yield strength is explained by using results of the present simulations and ab initio calculations of the total energy and Young's modulus for an infinite atomic chain.

A global A-state potential surface for H2O: Influence of excited states on the O(1D)+H2 reaction

Abstract: In this article a global potential energy surface for the 1A′′ state of H2O based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio results is presented. The resulting 1A′′ surface is used in conjunction with a previously determined 1A′ surface to study the O(1D)+H2(HD,D2) reaction dynamics, with emphasis on the influence of the 1A′′ excited state on measurable properties such as the reactive cross sections, rate coefficients, and product state distributions. There is a reactive threshold of about 2 kcal/mol on the 1A′′ surface, and even at 5 kcal/mol, the 1A′′ reactive cross section is only a small fraction (∼20%) of the barrierless 1A′. However, the 1A′′ surface populates very specific product vibrational states (v=3–4) and gives strongly backward peaked differential cross sections, so certain types of measurements are quite sensitive to the presence of this excited state. In particular, better agreement is found with experimental vibrational and angul...

A Combined EPR and Quantum Chemical Approach to the Structure of Surface Fs+(H) Centers on MgO

Abstract: Fs+(H) color centers at the surface of MgO have been studied using a combined EPR and quantum chemical approach. Fs+(H) are paramagnetic excess electrons centers where the unpaired electron is trapped in a surface anion vacancy. They are formed at the surface of thoroughly dehydrated MgO (1073K) upon UV irradiation under hydrogen in parallel with the formation of minor fractions of different color centers. The whole EPR spectrum resulting from irradiation has been analyzed by simulation of the experimental profile. Fs+(H) centers are characterized by an axial g tensor and display a hyperfine interaction with a hydrogen nucleus (belonging to an hydroxyl group stabilized nearby the vacancy) and two distinct families of 25Mg nuclei characterized by a large (10.5 G) and a small (0.7 G) hyperfine coupling constant, respectively. Both EPR and ab initio calculations on clusters of ions converge in indicating that the features of the centers are due to the polarization of the electron density by the positive char...

Hydrated proton clusters: Ab initio molecular dynamics simulation and simulated annealing

Abstract: An ab initio molecular dynamics simulation technique is developed employing the Born–Oppenheimer (BO) approach in the framework of a Gaussian implementation of Kohn–Sham density functional theory (DFT). Simulation results for H5O2+ at 200 K are reported. The density profiles, autocorrelation functions and power spectra are presented. The anharmonic frequencies at 200 K are found to be close to the harmonic frequencies calculated directly from quantum methods at 0 K. Structures of large hydrated proton clusters are optimized. Simulated annealing techniques were employed to search for low energy structures and found to be very useful for clusters with 7–8 water molecules. A few very different structures with ground state energy 1–2 kcal/mol apart are shown. H3O+ is found to be the central unit of a few structures optimized. The ionic hydrogen bond was responsible for the stability of the H9O4+ unit in the large hydrated proton clusters. We also find structures with nascent H5O2+ units at the center whose en...

A coupled-cluster ab initio study of triplet c3h2 and the neutral-neutral reaction to interstellar c3h

Abstract: For the initially formed C3H2 collision complexes of molecular beam experiments ab initio calculations are presented. Resolving energetics and properties of these intermediates is essential for the understanding of the reaction of C(3P) with C2H2 to form interstellar cyclic and linear isomers of C3H. Computed reaction energies agree with results from molecular beam experiments. The combination of crossed molecular beam experiments and ab initio calculations allows us to identify two reaction channels for the carbon–hydrogen exchange and to explain astronomical observations of a higher c-C3H to 1-C3H ratio in dark clouds as compared to hotter envelopes of carbon stars.

A new He-CO interaction energy surface with vibrational coordinate dependence. I. Ab initio potential and infrared spectrum

Abstract: The intermolecular potential energy surface of the He–CO complex including the CO bond length dependence has been calculated using symmetry-adapted perturbation theory (SAPT). The potential has a minimum of em=−23.734 cm−1 with Rm=6.53 bohr at a skew geometry (ϑm=48.4°) if the molecular bond length is fixed at the equilibrium value of 2.132 bohr. We have applied the potential in the calculation of bound state levels and the infrared spectrum for the 3He–CO and 4He–CO complexes. The computed ab initio transition frequencies are found to agree within 0.1 cm−1 with experiment. In paper II [J. P. Reid, H. M. Quiney, and C. J. S. M. Simpson, J. Chem. Phys. 107, 9929 (1997)], the potential surface is used to calculate vibrational relaxation cross sections and rate constants.

Shape and stability of quantum dots

Abstract: The formation of dislocation-free three-dimension- al islands by heteroepitaxial growth of lattice mismatched materials is utilized to produce partially ordered arrays of quantum dots. The equilibrium shape of these islands results from the competition between surface and elastic energies. We have studied the system InAs=GaAs.001/ in detail. InAs surface energies have been computed ab initio for several orientations, and the elastic energy of the islands has been cal- culated within a continuum theory. The resulting equilibrium islands are hills bounded by f110g, f111g ,a nd f N 1 N 1 N 1 gfacets and a .001/ surface on top. We compare to experiment and discuss the influence of growth kinetics on the shape.