Showing papers on "Ab initio quantum chemistry methods published in 1991"

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...

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.

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 ...

Ground-state correlation energies for two- to ten-electron atomic ions.

Abstract: Improved estimates of the nonrelativistic stationary nucleus correlation energies of the ground-state atomic ions with three to ten electrons and Z up to 20 are derived by combining experimental data and improved ab initio calculations. Unlike previous work in this area, we focus on the correlation contribution to individual ionization energies, computed by comparing experimental data with relativistic complete-valence-space Hartree-Fock energies.

Endohedral complexes: Atoms and ions inside the C60 cage

Abstract: The results of ab initio electronic structure calculations on the C60 cage and its endohedral (‘‘inside‐the‐cage’’) complexes with F−, Ne, Na+, Mg2+, and Al3+ are presented. Placing the ions at the center of the cage results in a net stabilization and screening of the charges. The ionic guests either decrease (F−) or increase (Na+, Mg2+, and Al3+ ) the cage radii. The complexes with the ions at the cage center are local maxima with respect to the displacement of the guests. The C60⋅Ne complex, which is destabilized by ca. 0.4 kcal/mol relative to the separated components, is an energy minimum. In the C60⋅Na+ complex, the energy minimum (which lies only 0.8 kcal/mol below the maximum) corresponds to the Na atom displaced by 0.66 A from the cage center. The calculated properties of the endohedral complexes are easily rationalized with a model involving a double‐layer polarizable C60 cage affected by the electrostatic potential produced by the enclosed guests.

Structural and electronic properties of sodium microclusters (n=2–20) at low and high temperatures: New insights from ab initio molecular dynamics studies

Abstract: We present the results of extensive computer simulations of several sodium microclusters, using the Car–Parrinello method (unified density‐functional theory and molecular dynamics). Dynamical simulated annealing strategies are adopted in the search for low‐energy minima of the potential energy surface. A detailed analysis of the results for both structural and electronic properties at temperatures in the 0–600 K range is carried out, which allows us for the first time to gain insight into the structural ‘‘growth’’ pattern, the extent of the validity of (spherical, spheroidal, and ellipsoidal) jellium models, and the effects of temperature. In particular, new and unforeseen structures are discovered for n=10, 13, 18, and 20 and we emphasize the constant presence of arrangements with local pentagonal symmetry for the low‐energy isomers as well as the similarity of the structural pattern with that of Lennard‐Jones systems. Shape transformations with increasing temperature are observed, ‘‘rigidity’’ and ‘‘nonrigidity’’ of the individual clusters examined, and the presence of distinct isomers is identified for the smaller ones. Closing of electronic shells is confirmed for Na8 and Na20 and—to a minor extent only—for Na18. Hybridization of cluster states of different angular momenta, which represents a deviation from the spherical shell model, is discovered in several cases and discussed in detail, also in correspondence with the presence of anisotropy of the electronic potential. In most cases, this hybridization is observed to increase with increasing temperature, in parallel with the increase of the eccentricity of the cluster shape. In spite of the relatively high atomic mobility, our results do not support a spherical liquid‐droplet picture for the atomic distribution.

Ab initio potential energy surfaces and trajectory studies of A‐band photodissociation dynamics: CH3I*→CH3+I and CH3+I*

Abstract: Ab initio contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of 3Q0 and 1Q1 excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six‐dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH3 product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH3 rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I‐channel product because the 1Q1 surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because 3Q0 still has a large bending force constant. The ca...

Nonlinear optical properties of p-nitroaniline: An ab initio time-dependent coupled perturbed Hartree-Fock study

Abstract: For p‐nitroaniline the ab initio method with a double‐zeta basis set which includes semidiffuse polarization functions has been used to calculate the dipole moment μ, frequency‐dependent linear polarizability α, and nonlinear hyperpolarizabilities β and γ using the time‐dependent coupled perturbed Hartree–Fock approach. The computation procedure used here yields information on the dispersion behavior of all the tensor components of polarizability and various hyperpolarizability terms. The largest dispersion effect is observed for the diagonal components of the polarizability and hyperpolarizability tensors along the long in‐plane axis. The magnitudes of the various hyperpolarizability terms which describe the various second‐order nonlinear processes show the following trend: β(−2ω;ω,ω) ≳β(0;ω,−ω)=β(−ω;0,ω) ≳β(0;0,0), with β(−2ω;ω,ω) exhibiting the largest frequency dispersion. The various second hyperpolarizability terms which describe the various third‐order nonlinear optical processes show the following trend: γ(−3ω;ω,ω,ω) ≳γ(−2ω;0,ω,ω) ≳γ(−ω;ω,−ω,ω) ≳γ(−ω;0,0,ω) ≊ γ(0;0,ω,−ω) ≳γ(0;0,0,0). Again γ(−3ω;ω,ω,ω) shows the largest dispersion effect. The results of existing semiempirical calculations on p‐nitroaniline are compared with that of the present ab initio calculation, and the problem due to the arbitrary parametrization procedure adopted in the past for semiempirical calculation is discussed. The computed values of the first resonance energy, the dipole moment, and the polarizability are in good agreement with the respective values experimentally observed, within the spread of the existing experimental data. In contrast, the computed β and γ values are considerably smaller than the respective experimentally determined values. We attribute this discrepancy to two sources. First, in the theoretical calculation electron correlation has been neglected, and the basis set used, although large, may not still be adequate. Second, there is a considerable spread in the reported experimental values for a given nonlinear coefficient making any comparison between the theory and the experiment difficult.

Analytic gradients for coupled-cluster energies that include noniterative connected triple excitations: Application to cis- and trans-HONO

Abstract: An efficient formulation of the analytic energy gradient for the single and double excitation coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations is presented. The formulation has a small computational cost, and the algebraic manipulations may be applied generally to the analytic gradient of Moller-Plesset perturbation theory energies. The new formulation has been implemented in an efficient set of programs that utilize highly vectorized algorithms and has been used to investigate the equilibrium structures, harmonic vibrational frequencies, IR intensities, and energy separation of cis- and trans-HONO.

Nonclassical double-bridged structure in silicon-containing molecules: Experimental evidence in Si2H2 from its submillimeter-wave spectrum.

Abstract: The first spectroscopic evidence of a double-bridged silicon molecule is reported. The submillimeterwave rotational spectrum of the disilyne Si 2 H 2 has been observed in a low-power silane plasma cooled at liquid-nitrogen temperature, with argon as buffer gas. These measurements led to the determination of the r 0 structure, which is in good agreement with ab initio calculations. This nonclassical structure is confirmed by the observation of the 29 Si and 30 Si monosubstituted forms

Electronic structures of the negative ions Si−2 –Si−10: Electron affinities of small silicon clusters

Abstract: Accurate ab initio calculations have been performed to investigate the structures and energies of the negative ions of Si2–Si10. The effects of polarization functions, diffuse functions, and electron correlation have been included in these calculations. In most cases, there is a good correspondence between the ground state structures of the negative ions and those of the corresponding neutral species. Adiabatic electron affinities are computed and compared with recent experimental measurements. Si3, Si5, Si8, and Si9 are found to have electron affinities which are larger than their neighbors. This result is interpreted using our previous calculations on the low‐lying states of the corresponding neutral species.

A reaction surface Hamiltonian treatment of the double proton transfer of formic acid dimer

Abstract: The double proton transfer reaction of the isolated formic acid dimer has been investigated within the reaction surface Hamiltonian framework, using a newly calculated three‐dimensional ab initio potential energy surface. The symmetric (synchronous) proton movement, the asymmetric (asynchronous) proton movement and the relative motion of two formic acid molecules have been explicitly included in the calculation. The calculation gives a tunneling splitting of 0.004 cm−1, which is considerably smaller than a previous theoretical prediction (0.3 cm−1). An effective tunneling path has been calculated from the lowest vibrational eigenfunction of the reaction surface Hamiltonian, and the path deviates significantly from the minimum energy path on the potential energy surface. The new results are consistent with the conventional understanding of heavy–light–heavy mass combination reactions. The effective reaction path from this calculation reveals evidence of asymmetric proton movement. However, a synchronous do...

Ab initio study of intermolecular potential of H2O trimer

Abstract: Nonadditive contribution to the interaction energy in water trimer is analyzed in terms of Heitler–London exchange, SCF deformation, induction and dispersion nonadditivities. Nonadditivity originates mainly from the SCF deformation effect which is due to electric polarization. However, polarization does not serve as a universal mechanism for nonadditivity in water. In the double‐donor configuration, for example, the Heitler–London exchange contribution is the most important and polarization yields the wrong sign. Correlation effects do not contribute significantly to the nonadditivity. A detailed analysis of the pair potential is also provided. The present two‐body potential and its components are compared to the existing ab initio potentials (MCY) as well as to empirical ones (RWK2,TIP,SPC). The ways to improve these potentials are suggested.

Direct calculation of electron density in density-functional theory: Implementation for benzene and a tetrapeptide

Abstract: A recently developed approach for the direct calculation of electron density is implemented for polyatomic molecules: benzene and a tetrapeptide with four glycine residues The method uses the density as the basic variable, divides a system into subsystems, and determines the density for each subsystem It is found that the method is capable of describing the electronic structure with accuracy comparable to the Kohn-Sham method This substantiates the hope for ab initio calculations of large systems beyond the reach of conventional methods

Theoretical characterization of tetrahedral N4

Abstract: Results are reported from ab initio theoretical calculations of the structure and energetics of tetrahedral N4. The theoretical basis and implementation of the computations are reviewed, and the results are presented in tables. It is found that a tetrahedral N4 molecule has energy 186 kcal/mole greater than that of two N2 molecules, with a 61-kcal/mole barrier between the two species. Brief consideration is given to the possible use of N4 (should a process for synthesizing it be developed) as a high-energy-density fuel for supersonic transport aircraft.

Solvent and secondary kinetic isotope effects for the microhydrated SN2 reaction of Cl-(H2O)n with CH3Cl

Abstract: We have calculated gas-phase rate coefficients and deuterium kinetic isotope effects (KIEs) for isotopic substitution in either the methyl group or the water of the title reaction with n=1 and 2. The calculations are carried out by variational transition-state theory with semiclassical transmission coefficients, and they are based on 27- and 36-dimensional reaction-path potentials presented previously. A critical aspect of the potentials is that the solute part is based on high-level ab initio calculations. We also analyze the effect of deuterium substitution at methyl for the case of n=0. We calculate an inverse effect for substitution at methyl both for bare solute (n=0) and for microhydrated solute with n=1 or 2. A detailed mode analysis shows that the inverse effect for the unhydrated reaction is dominated by C-H stretch contributions rather than than by CH 3 deformations as is usually assumed in analyzing experimental data on solution-phase reactions

Anharmonic vibrational properties of CH2F2 : A comparison of theory and experiment

Abstract: Ab initio theoretical chemistry is used to provide a complete understanding of the infrared spectroscopy of CH2F2. Second‐order Mo/ller–Plesset perturbation theory (MP2) with a 631G extended basis set is used to provide a quartic expansion of the potential energy surface and a cubic expansion of the dipole surface. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for fundamentals, selected overtones, and combination bands. Effects of Fermi resonance, Darling–Dennison resonance, and Coriolis resonance are included by matrix diagonalization. Empirical (x,K) relations are used to demonstrate that the anharmonic constants for C–H are in good agreement with those determined from CH2Cl2. The local mode nature of the CH overtones is demonstrated. Important resonances are found to be (ν3,2ν4), (ν8,ν4+ν9), and (ν1,2ν2,2ν8,ν4+ν8+ν9, 2ν4+2ν9,ν3+2ν9). Rotational constants, quartic and sextic centrifugal distortion constants, vibration rotation interaction consta...

The ground and excited states of C60M and C60M+ (M=O, F, K, Ca, Mn, Cs, Ba, La, Eu, U)

Abstract: Restricted Hartree–Fock ab initio calculations using relativistic core potentials were performed on C60M (M=O, F, K, Ca, Mn, Cs, Ba, La, Eu, U) complexes with M as the central atom in the C60 truncated icosahedron. The icosahedral symmetry was used to great advantage in the calculations. The ground and excited states of both neutral complexes and their positive ions were studied, and the population analyses for the ground states of the complexes were obtained. The C60 cage accepts one or two electrons from electropositive elements in a formal sense, but the actual charge is usually less. Electrons in large‐radius s orbitals on the central atom tend to move outward to the carbon cage or inward to smaller‐radius d orbitals on the central atom. For the larger central atoms, ionization occurs from a cage orbital so that the ionization potentials of these complexes are almost constant.

Dielectric relaxation dynamics of water and methanol solutions associated with the ionization of N,N-dimethylaniline : theoretical analyses

Abstract: The solvation dynamics associated with the ionization of N,N‐dimethylaniline (DMA) in water and methanol solutions has been studied theoretically. Potential energy surfaces of DMA and DMA+ were computed by ab initio molecular orbital (MO) methods. Intermolecular pair potential functions between DMA and H2O were developed with the aid of the electron distributions of DMA and H2O and the results of MO calculations for the DMA–H2O system. Potential functions between DMA and MeOH were also determined empirically using the parameters for DMA–H2O interaction. Equilibrium and nonequilibrium molecular dynamics calculations were carried out for the DMA–water and DMA–methanol solutions. The simulation results were analyzed comparing two solvents in order to obtain a realistic molecular model for the solvation dynamics of DMA in polar solvents. The solvation coordinate was defined by the potential energy difference between neutral and cation states and free energy curves along it were constructed using the umbrella ...

The wave packet motion and intramolecular vibrational redistribution in CHX3 molecules under infrared multiphoton excitation

Abstract: We report results from quantum dynamical simulations of ultrafast vibrational redistribution processes in the CH chromophore of CHX3 molecules (CHD3, CHF3) during and after infrared‐multiphoton excitation. The vibrational Hamiltonian is based on results from high resolution spectroscopy and ab initio calculations of the potential hypersurfaces for these molecules. The quantum dynamical calculations involve accurate solutions of the time dependent quantum equations of motion by means of both Floquet and quasiresonant approximations. We find mode selective redistribution between the CH stretching and bending modes on a time scale of 50 to 100 fs. Other modes participate only on much longer time scales (>1 ps), as was shown previously by analysis of the spectra. For the real, strongly anharmonic systems (k’sbb≂30 to 100 cm−1 ), the redistribution is nonclassical with fast spreading to a quasimicrocanonical distribution, which is particularly pronounced if a narrow range of energies (for example, the N=6 poly...

Calculation of the Si–H bond energies for the monohydride phase of Si(100)

Abstract: Ab initio calculations are carried out on Si9H12, Si9H13, and Si9H14 clusters, chosen to model the Si(100)–(2×1) reconstructed surface and its hydrides. A value of 56 kcal/mol is obtained for the energy of the recombinative hydrogen desorption. The energies required to remove the first and second H atoms from a doubly‐occupied site are 81 and 76 kcal/mol, respectively.

Ab initio model potential study of the equilibrium geometry of alkaline earth dihalides: MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I)

Abstract: The ground state equilibrium geometry of alkaline earth dihalides MX2 (M=Mg, Ca, Sr, Ba; X=F, Cl, Br, I) has been optimized at the Hartree–Fock (HF) level using the Cowan–Griffin relativistic ab initio model potential method and a uniformly good, extended, spd valence basis set. The results show that, according to the method, all magnesium dihalides and CaCl2, CaBr2, and CaI2 are linear, SrF2 and all barium dihalides are bent, and CaF2 and SrCl2, SrBr2, and SrI2 are quasilinear molecules. The alkaline earth (n−1)d orbitals are shown to be responsible for the bending of the heavier molecules while their (n−1)p orbitals contribute considerably to the final quantitative prediction of the apex angle and the relative stability of the bent structures. Relativistic effects are shown to be very small on the bond distances and vibrational frequencies; they are important on the size of the bending barrier of the bent molecules. The results obtained are compared to previous theoretical studies and provide some insig...

Ab initio quantum chemical calculations of aluminum substitution in zeolite ZSM-5

Abstract: The authors have performed ab initio quantum mechanical calculations in monomeric clusters modeling the 12 different T sites of zeolite ZSM-5. By comparing the results of calculations that use minimum basis sets with those that employ valence double-[zeta] bases, the authors conclude that minimum basis sets are unreliable for predicting relative replacement energies for the substitution of silicon by aluminum atoms at the T sites of the zeolite. From these calculations, it is also concluded that small differences in the bond lengths and angles can significantly alter the order of the sites with respect to the replacement energies. From calculations using valence double-[zeta] basis sets on T(OH)[sub 4] monomers, it is concluded that in the absence of protons or other ions, the most favorable sites for Al substitution in zeolite ZSM-5 are the T[sub 6], T[sub 12], and T[sub 9] sites, whereas the least favorable site is T[sub 3]. However, the least favorable and most favorable sites only differ by 3.3 kcal/mol. The authors also present a simple empirical model that is capable of reproducing the results of the ab initio calculations. This model gives the replacement energy in terms of the bond lengths and bond angles about each site.

Van der Waals Complexes in 1,3-Dipolar Cycloaddition Reactions: Ozone-Ethylene

Abstract: Microwave spectra of O3-CH2=CHz, O3-CDz=CHz, 03-trans-CHD=CHD, and O,-cis-CHD=CHD have been observed with a pulsed-beam Fabry-Perot cavity, Fourier transform microwave spectrometer. Internal motions in the van der Waals complex give two states for the normal, 1 ,I-dideuterated and trans-I ,2-dideuterated isotopic forms. The C-type transitions of the two states for the isotopic species above, as well as the one observed isotopic form of 03-cis-CHD=CHD, independently fit to an asymmetric top Watson Hamiltonian. Stark effect measurements for 03-CHz=CHz giye wLa = 0.017 (I) D and p, = 0.466 (2) D. The microwave data are only consistent with a structure having C, symmetry in which the nearly parallel planes of ethylene and ozone have a center of mass separation of R,, = 3.290 (3) A. Ab initio calculations at the MP4 level indicate that the preferred geometry corresponds to small tilts of the ozone and ethylene planes, which place an exo-oriented pair of hydrogens toward the terminal oxygens of ozone. Both the theoretical and microwave results suggest the tunneling splitting arises at least in part from a 180' rotation of ethylene about its Cz axis, which is perpendicular to the ethylene plane. I ,3-Dipolar cycloaddition theory, orbital symmetry rules, and ab initio calculations of the complex and transition states are used to argue that 03-CH2=CH2 lies in a shallow minimum on the reaction coordinate prior to the transition state in the reaction of ozone plus ethylene, which produces the primary product 1,2,3-trioxolane.

Theoretical studies of inorganic and organometallic reaction mechanisms. 2. The trans effect in square-planar platinum(II) and rhodium(I) substitution reactions

Abstract: Ab initio calculations with an effective core potential have been used to study the mechanism of substitution reactions for square-planar transition-metal complexes. Pseudo-trigonal-bipyramidal transition states with rather small entering-ligand to metal to leaving-ligand angles were found for the substitution reactions investigated in this paper. The stability of the transition state is determined by both σ and π effects of the ligands

The oh vibrational-spectrum of liquid water from combined abinitio and monte-carlo calculations

Abstract: The infrared vibrational OH stretching spectrum of isotopically isolated HDO molecules in liquid water has been calculated by ab initio methods at the MP2 level for a number of geometrical configurations taken from a Monte Carlo simulation. Each vibrating water molecule with its environment was described by a pentamer supermolecule, surrounded by a large number of point charges representing polarized water molecules. The anharmonic stretching potentials (MP2 force constants up to fifth order) for 40 uncoupled OH water vibrators were calculated. The average computed re distance found for liquid water is 0.01 A longer than the free‐water value. The frequencies were obtained by solving the one‐dimensional Schrodinger equation variationally for each OH potential curve. Using the squared dipole moment derivatives, which vary by a factor of 7 over the frequency band, the density‐of‐states histograms were converted to intensities. The resulting computed average frequency downshift is ∼260 cm−1, compared to ∼310 ...