Showing papers on "Ab initio quantum chemistry methods published in 1988"

A complete basis set model chemistry. I. The total energies of closed‐shell atoms and hydrides of the first‐row elements

Abstract: The major source of errror in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. A complete basis set model chemistry is defined to include corrections for basis set truncation errors. This model uses double zeta plus polarization level atomic pair natural orbital basis sets to calculate molecular self‐consistent‐field (SCF) energies and correlation energies. The small corrections to give the complete basis set SCF energies are then estimated using the l−6 asymptotic convergence of the multicenter angular momentum expansion. The calculated correlation energies of the atoms He, Be, and Ne, and of the hydrides LiH, BH3, CH4, NH3, H2O, and HF, using the double zeta plus polarization basis sets vary from 83.0% to 91.2% of the experimental correlation energies. However, extrapolation of each of the pair energies and pair‐coupling terms to the complete basis set values using the asymptotic convergence of pair natural orbital expansions retrieves from 99.5±0.7% to ...

Ab initio analytic polarizability, first and second hyperpolarizabilities of large conjugated organic molecules: Applications to polyenes C4H6 to C22H24

Abstract: The static dipole polarizability and second hyperpolarizability tensors are calculated for polyene systems via ab initio coupled‐perturbed Hartree–Fock theory. The effect of basis set augmentation on the calculated properties is explored for C4H6 and example basis sets are used to calculate the polarizability and second hyperpolarizability for the longer polyenes: C6H8, C8H10, C10H12, C12H14, C14H16,C16H18, C18H20, C20H22, C22H24. Results for the finite polyenes are extrapolated to predict the unit‐cell polarizability and second hyperpolarizability of infinite polyacetylene. The working equations which take advantage of the 2n+1 theorem of perturbation theory for calculating up to the second hyperpolarizability are given, and their implementation is briefly discussed. In particular it is shown that the implementation is readily amenable to parallel processing.

Bonding and stabilities of small silicon clusters: A theoretical study of Si7–Si10

Abstract: Ab initio calculations have been performed to study the structures and energies of intermediate‐sized silicon clusters (Sin, n=7–10). All geometries have been optimized at the Hartree–Fock (HF) level of theory with the polarized 6‐31G* basis set. The harmonic vibrational frequencies have been evaluated at the HF/6‐31G* level of theory. Electron correlation effects have been included by means of fourth order Mo/ller–Plesset perturbation theory. The most stable structure for Si7 is a pentagonal bipyramid and the lowest energy calculated structures for Si8–Si10 correspond to capped octahedral or prismatic geometrical arrangements. The evolution of the cluster geometries with increasing size is discussed. Clusters containing four, six, seven, and ten atoms have been identified as ‘‘magic numbers’’ for small silicon clusters, both theoretically and experimentally. The hybridization and bonding in small silicon clusters is discussed. Our results are used to interpret the recent photoelectron spectra of negative...

The ab initio model potential representation of the crystalline environment. Theoretical study of the local distortion on NaCl:Cu+

Abstract: In this paper we formulate a model potential approach to take into account the crystalline environment within the Hartree–Fock–Roothaan formalism. The formulation is based on the assumption that the theory of separability of many‐electron systems may be applicable to the group of electrons within a reference cluster and the groups of electrons on a set of external lattice sites which, in turn, can be represented according to the ab initio model potential method. The characteristics of the model potentials permit to analyze the contributions to the cluster energies and wave functions of different environmental effects, such as point‐charge and charge‐density Coulomb interactions and quantum interactions (exchange and orthogonality). The formalism is applied to the SCF calculation on the ground state of the octahedaral CuCl5−6 cluster (all‐electron calculation) embedded in a NaCl lattice which is represented by 118 model‐potential ions and 604 point‐charge ions. The calculation reveals that (i) the quantum ...

A systematic study of polarizability and microscopic third‐order optical nonlinearity in thiophene oligomers

Abstract: A systematic study of the dependence of the band gap, the linear optical susceptibility, the polarizability α, and the second hyperpolarizability γ, on the number of repeat unit is conducted for the thiophene series from monomer to hexamer. The linear optical susceptibilities for oligomers have been determined from the refractive index measurements on vacuum deposited films using the m lines technique. The orientationally averaged polarizabilities 〈α〉 have been measured from refractive index measurements of THF solutions. The orientationally averaged second hyperpolarizabilities 〈γ〉 have been measured by degenerate four‐wave mixing studies of THF solutions. The validity of the Lorentz–Lorenz approximation is tested and found to be satisfactory. The experimental values of 〈α〉 and 〈γ〉 for thiophene and 〈α〉 for bithiophene are found to be in qualitative agreement with those obtained by a recent ab initio calculation which used the finite field method and included diffuse polarization functions. The experimentally observed dependence of 〈α〉 and 〈γ〉 on the number N of the thiophene repeat unit is compared with that predicted by a free electron model, PPP methods, and the ab initio calculations. For the case of polarizability, the repeat unit dependence is in good agreement with that predicted by the ab initio calculations but for the case of second hyperpolarizability, the agreement is not as good.

Collision-induced dissociation and ab initio studies of boron cluster ions: determination of structures and stabilities

Abstract: Bonding in small boron cluster cations (BZTl3+) is examined by measurement of appearance potentials and fragmentation patterns for collision-induced dissociation (CID) with Xe. Cluster stabilities are generally found to increase with increasing cluster size; however, there are large fluctuations from the overall trend. The lowest energy fragmentation channel for all size cluster ions is loss of a single atom. Clusters smaller than six atoms preferentially lose B', while for the larger clusters the charge remains on the BW1+ fragment. The results are used to estimate cluster ionization potentials and geometries. Comparison of measured stabilities with "magic" numbers in the cluster ion size distribution and with total CID cross sections shows that neither is a reliable indicator of stability. We also report on ab initio calculations for both neutral and ionic BI4. The results include cluster geometries, ionization potentials, charge distributions, dissociation energies, and bonding character. The results for IPS, geometries, and De)s are compared with experiment.

Interplay of Jahn–Teller and pseudo‐Jahn–Teller vibronic dynamics in the benzene cation

Abstract: The static and dynamic aspects of the vibronic interaction of the B 2E2g and C 2A2u electronic states of C6H+6 are analyzed. In the approximation of linear vibrational and vibronic coupling, the model Hamiltonian for this system comprises eight nonseparable vibrational modes, six of which are degenerate (two of A1g symmetry, four of E2g symmetry, and two of E2u symmetry). The coupling constants are estimated from existing ab initio SCF and semiempirical (CNDO/S) calculations. The topology of the adiabatic potential‐energy surfaces of this class of model Hamiltonians is investigated. It is shown that the model exhibits a variety of conical intersections which dominate the vibronic dynamics. The dynamical problem is solved with simultanteous inclusion of six vibrational modes, four of which are degenerate (the Jahn–Teller coupling of two of the E2g modes is negligible). Hamiltonian matrices with dimensions up to 6×106 are diagonalized using the Lanczos algorithm. After some adjustments of coupling constan...

A DIM model for homogeneous noble gas ionic clusters

Abstract: The method of diatomics-in-molecules (DIM) is applied to the calculation of the energy of the homogeneous noble-gas ionic clusters Ar + and Xe + forn=3, 4, ..., 22. The trimers are stable symmetric linear molecules exhibiting chemical binding, a result in agreement both with ab initio calculations and with previous DIM work. The clusters up ton=13 are best described as a trimer ion surrounded by neutrals, whereby the charge distribution changes slightly with increasingn. Both noble gases exhibit a special stability associated with the completion of the first shell of neutral atoms atn=13. Asn increases from 13 to 22, there is a greater delocalization of the positive charge, the central ion tending to become a linear tetramer, symmetric for Xe and unsymmetric for Ar. Energies of the excited electronic states are reported and the possibility of developing simpler DIM models for the clusters and for mixed noble gases is discussed.

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

Abstract: We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A state and a calculated SCF dipole moment function for the X→A transition. The dynamical calculations are performed within the infinite‐order‐sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A state potential energy surface and details o...

Interaction potentials for alkali ion–rare gas and halogen ion–rare gas systems

Abstract: The Tang–Toennies model [J. Chem. Phys. 80, 3725 (1984)] has been modified to predict the potentials for ion–atom systems. First order SCF energies are used to describe the repulsive potential. The long range second order induction and dispersion potential terms up to R−10 are either taken from ab initio calculations or estimated and each term is appropriately damped. The potentials for Li+, Na+, K+, F−, and Cl− interacting with He, Ne, and Ar are found to agree well with both theoretical and experimental data within the expected errors. For comparison with the model new ab initio calculations have been performed for Na+–Ar and the results are in excellent agreement with the model predictions (<10%).

A realistic double many-body expansion (DMBE) potential energy surface for ground-state O3 from a multiproperty fit to ab initio calculations, and to experimental spectroscopic, inelastic scattering, and kinetic isotope thermal rate data

Abstract: A new single-valued potential energy surface is reported for the ground electronic state of ozone from the double many-body expansion (DMBE) method. The parameters appearing in the DMBE formalism are determined from a multiproperty analysis using ab initio energies, and experimental data from spectroscopic, incomplete total scattering cross section, and kinetic thermal rate measurements. Based on this new surface, thermal rate coefficients for the 18O + 16O2 → 18O16O + 16O isotope exchange reaction are also reported over the temperature range 300 < T < 2000 K from the quasi-classical trajectory method.

Bonding in GaAs

Abstract: Experimental measurements of structure factors in GaAs have been compared with the results of ab initio calculations by the pseudopotential method. We measure the charge in the bond from charge-density maps to be 0.071 electron, in good agreement with theory. The measurements were obtained by an electron-diffraction technique which can be applied to submicron crystals, thus greatly extending the range of materials for which structure factors can be measured, and theoretical calculations tested.

Molecular dynamics study of an aqueous SrCl2 solution

Abstract: A molecular dynamics simulation of a 1.1 m SrCl/sub 2/ solution was performed with an improved central force model for water at the experimental density at room temperature. The ion-water and ion-ion potentials were derived from ab initio calculations. The simulation extended over 4 ps at an average temperature of 298 K. The structural properties of the solution are discussed on the basis of radial distribution functions and the orientation of the water molecules and their geometrical arrangement in the hydration shells of the ions. The dynamical properties are calculated from various autocorrelation functions. Results are presented for the influence of the ions on self-diffusion coefficients, hindered translations, librations, and internal vibrations of the water molecules.

An analytical six‐dimensional potential energy surface for (HF)2 from ab initio calculations

Abstract: We have developed an analytical expression for the 6D potential energy surface of (HF)2 and fitted it to 1061 ab initio points covering an energy range of up to 25 000 cm−1 above equilibrium. The ab initio calculation used the coupled pair functional approach with an extended polarized basis set. We have adjusted 42 parameters (and constrained 7 others) in the fitting of the analytical function to the points; the standard deviation of the weighted fitting is 26.8 cm−1 . The minimum energy path from the linear saddle point (345 cm−1 above equilibrium), through the equilibrium point, to the C2h saddle point (332 cm−1 above equilibrium) has been determined, and several cuts through the potential are presented. We plan to use this potential surface in reaction path and close coupling calculations of rotation–vibration term values.

Correlated ab initio harmonic frequencies and infrared intensities for furan, pyrrole, and thiophene

Abstract: Equilibrium geometries, harmonic vibrational frequencies, and infrared intensities are calculated analytically at the second-order Moeller-Plesset level (MP2) with a DZP basis for the five-membered heterocyclic aromatics furan, pyrrole, and thiophene. The results are of an accuracy to show up misassignments in the original experimental interpretation of the spectra. They also give confidence that ab initio calculations including electron correlation and using flexible basis sets can describe accurately the quadratic part of the potential energy surface. For such systems, these ab initio studies will aid the spectroscopic determination of force constants.

Gaussian basis sets for high-quality ab initio calculations

Abstract: Different types of Gaussian basis sets for accurate LCAO calculations are discussed. For calculations designed to recover a substantial portion of the correlation energy, we suggest the use of basis sets comprising natural orbitals from correlated calculations on the atoms. These basis sets have proven to be very efficient in accounting for large fractions of the molecular correlation energy. For cases in which an SCF or MCSCF treatment is adequate the use of a floating basis provides a rapid convergence to the large-basis limit.

Ab initio calculations of correlation effects in trans‐polyacetylene

Abstract: The fundamental band gap and the exciton binding energies of t‐PA have been evaluated with the aid of ab initio Hartree–Fock plus Mo/ller–Plesset perturbation theoretical calculations. The irreducible self‐energy part has been expanded up to third order and the irreducible vertex part has been expanded up to second order. The results support one of several controversial assignments of the ∼2 eV peak in the experimental absorption spectrum of trans‐polyacetylene, namely that to interband transitions.

Infrared and Raman spectra of trans,trans-1,3,5,7-octatetraene and normal-coordinate analysis based on ab initio molecular orbital calculations

Abstract: The infrared and Raman spectra of trans,trans‐1,3,5,7‐octatetraene have been observed. It has been found that the spectral data reported in 1959 for this compound contain bands due to either impurities or geometrical isomers other than the trans,trans form. Band assignments are performed by using the results of ab initio molecular orbital (MO) calculations with the 6‐31G basis set. Important features of the vibrational modes and vibrational force field are discussed in some detail.

Structure, vibrational spectra, and IR intensities of polyenes from ab initio SCF calculations

Abstract: Ab initio SCF calculations are presented which deal with the structure of polyenes, as model systems for polyacetylene (PA). We used a double zeta quality basis set, and determined the structure of clusters up to C22H24 as well as the vibrational spectra of clusters up to C12H14, including infrared intensities. All of these data are analyzed by comparing with experimental information available on polyacetylene and previous theoretical studies. Specifically, we identified the vibrational modes corresponding to the observed absorption bands of PA. We comment on controversial points such as the relative contributions of internal coordinates to these modes, on the magnitude of interaction force constants, and also on the effect of light polarization on the absorption of some vibrational modes. We establish that the inclusion of polarization function in the basis set does not lead to significant changes in structural or energetic properties. Convergence of the cluster properties toward the 1D polymer properties was also tested with ab initio crystal orbital calculations.Ab initio SCF calculations are presented which deal with the structure of polyenes, as model systems for polyacetylene (PA). We used a double zeta quality basis set, and determined the structure of clusters up to C22H24 as well as the vibrational spectra of clusters up to C12H14, including infrared intensities. All of these data are analyzed by comparing with experimental information available on polyacetylene and previous theoretical studies. Specifically, we identified the vibrational modes corresponding to the observed absorption bands of PA. We comment on controversial points such as the relative contributions of internal coordinates to these modes, on the magnitude of interaction force constants, and also on the effect of light polarization on the absorption of some vibrational modes. We establish that the inclusion of polarization function in the basis set does not lead to significant changes in structural or energetic properties. Convergence of the cluster properties toward the 1D polymer propertie...

Ab initio investigation of the bound rovibrational states in the electronic ground state of HeN+2

Abstract: The two‐dimensional interaction potential of the N+2(X 2Σ+g) –He(X 1S) system has been calculated using highly correlated ab initio MCSCF‐CI wave functions for a fixed value of the N+2 bond length (2.110 a0). It is found to have a minimum about 140 cm−1 below the N+2 +He dissociation limit, significantly deeper than the minimum in the neutral system N2–He. This well depth is enough to give rise to a cluster‐like, bound structure with a considerable number of vibration–rotation levels in the electronic ground state. The well depth is almost independent of the N2–He angle, which leads to large amplitude bending motions. Rovibrational calculations have been performed on the surface for J=0, 1, and 2. Values for the rotational constant B, the fundamental stretching frequency νs, and its first and second overtones, and the rotational constant Cn in each nνs manifold, have been obtained from the energy levels computed. It is found that B=1.879 cm−1, 1νs =55.231 cm−1, 2νs =83.404 cm−1, 3νs =92.491 cm−1, C0=0.477...

Abinitio calculations of the electronic structure of small Nan, Na+n, Kn, and K+n clusters (n≤6) including core–valence interaction

Abstract: The electronic structure of small Nan, Na+n, Kn, and K+n alkali clusters (n≤6) is investigated using nonempirical core pseudopotentials, configuration interaction within a 3s/1p/1d Gaussian basis set per atom and including the core–valence interaction through a perturbative treatment. Equilibrium geometries, stabilities, ionization potentials, and fragmentation channels are derived and the role of electronic correlation in small alkali clusters is examined.

Ab initio calculations on the energy of activation and tunneling in the automerization of cyclobutadiene

Abstract: Results of ab initio two‐configuration CI‐SD/[3s2p1d/2s], MBPT(4), CCSD+T(CCSD), and CCSDT‐1 calculations are reported for the rectangular D2h equilibrium and square D4h transition structures of cyclobutadiene. The latter is a classic example of a multireference correlated method. The optimum CC and CH bond lengths found for the D4h transition structure are 1.448 and 1.093 A, respectively. The activation barrier for the automerization is 9.0 kcal/mol at the two‐reference GVB‐CISD level while the single reference CCSD gives 19.9, 14.4 for CCSD+T(CCSD) and finally a dramatic change to 9.5 at the highest CCSDT‐1 level. The importance of triples in overcoming the multireference character at the transition state is apparent. On the other hand, GVB‐CISD is simpler than CCSDT‐1 which attests to the importance of a qualitatively correct multireference starting point for this example. A less sophisticated computational method, GVB/4‐31G, which also gives a reasonable barrier of 10.2 kcal/mol was used for the const...

Abinitio MRD CI investigation of the optical spectra of C4 and C5 clusters

Abstract: The ground and excited electronic states of linear and rhombic C4 and linear C5 clusters have been studied with ab initio single reference and multireference configuration interaction calculations. The spectrum of linear C4 is characterized by the existence of low‐lying Π states at 1–1.5 eV above the 3Σ−g ground state. In rhombic C4, which has very similar ground state energy as the linear form, the first allowed transition is found at 2.4 eV. The optical spectrum of linear C5 exhibits some similarities with that of linear C3: in both molecules the ground state is 1Σ+g and the lowest allowed transition, 1Πu←1Σ+g, is about 3 eV above the ground state.

Ab initio configuration interaction study of the electronic and geometric structures of small sodium cationic clusters

Abstract: The favorable geometries of small sodium clusters Na+n (n=3–9) are determined with the analytical gradient method in the framework of the ab initio SCF approximation. The transition from the planar towards three‐dimensional cluster geometries is understood in terms of some basic quantum theoretical concepts. The binding energies per atom for Na+n clusters calculated with the MRD CI procedure increase, in general, as a function of n. Nevertheless, the atomization energy per atom as well as the ionization potential as functions of the nuclearity n exhibit well developed oscillations for even–odd n. Consequently, the fragmentation energy for the channel Na+n→Na+n−1+Na shows strong oscillations as well. This explains large abundances of cationic clusters with odd nuclearity found in some detection devices. The possible consequences for the fragmentation process of Na+n is discussed. It is found that the process Na+n→Na+n−2+Na2 is favorable for Na+5, Na+7, and Na+9.

Ab initio calculations of the olefin strain energies of some pyramidalized alkenes

Abstract: Discussion de la pyramidalisation sur les geometries optimisees et sur les energies des orbitales HOMO et LUMO

Stoichiometry and surface reconstruction: An ab initio study of GaAs(100) surfaces.

Abstract: Ga- and As-terminated GaAs(100) surfaces are studied through first-principles density-functional pseudopotential calculations of the energies of surfaces with different stoichiometries and reconstructions. We predict structural transitions as a function of Ga and As chemical potentials within an experimentally accessible range which is shown to be fixed by the bulk energies of Ga, As, and GaAs. The bulk energies are determined accurately by the ab initio calculations. The predicted surface stoichiometries are in good agreement with experimental data.

Three-body-exchange interaction in dense rare gases.

Abstract: Self-consistent phonon calculations are employed to demonstrate the importance of the three-body-exchange interaction in dense rare-gas solids. A Slater-Kirkwood form is adopted, the parameters of which are adjusted on ab initio calculations for helium and argon and calculated by a scaling law for krypton and xenon. When used in conjunction with realistic (Aziz) pair potentials and the Axilrod-Teller terms, the exchange interaction brings the calculated equation of state for each rare gas into excellent accord with experiment. This type of interaction should stabilize the hcp structure for all rare gases below 90 GPa.