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The N + N[SUB2] exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground [SUP4]A″ state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N + N[SUB2] (J = 0). The J-K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasiclassical calculation using a London-Eyring-Polanyi-Sato PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate three-dimensional quantal dynamics study for the N + N[SUB2] reaction system and the ab initio PES reported here is the first such surface for N[SUB3]. [ABSTRACT FROM AUTHOR]

Matrix infrared spectra of Co[SUB2]CO were obtained from cobalt vapors condensed with different isotopically substituted mixtures of carbon monoxide diluted in argon at 10 K. New bands at 1953.3, 488.7, 357.9, and 270.9 cm[SUP-1] are assigned to the v[SUBC-O], v[SUBM-CO], v[SUBM-C-O], and v[SUBM-M] fundamentals, respectively. Computations performed using density functional theory with the generalized gradient approximation (DFT-GGA) for the exchange-correlation potential predict the lowest energy state of Co[SUB2]CO to be [SUP5] A″ with the angle Co-Co-C of 116°. Our DFT-GGA vibrational frequencies of this state agree with the experimental frequencies extremely well--within 15 cm[SUP-1]. Our estimate for the Co[SUB2]-CO bond strength is 1.60 eV. Computed vibrational frequencies for Co[SUB2]CO[SUP+] and Co[SUB2] CO[SUP-] are included for comparison. [ABSTRACT FROM AUTHOR]

Compares the electronic structures of zinc oxide through spectroscopic analysis and density functionals. Related studies on zinc oxide spectra; Description of zinc oxide electronic states; Results of the spectroscopic constant calculations.

The bond dissociation energies of CH4, C2H2, and their fragments are investigated using basis set extrapolations and high levels of correlation. The computed bond dissociation energies (De) are accurate to within 0.2 kcal/mol. The agreement with the experimental (D0) values is excellent if we assume that the zero-point energy of C2H is 9.18 kcal/mol. The effect of core (carbon 1s) correlation on the bond dissociation energies of C–H bonds is shown to vary from 0.2 to 0.7 kcal/mol and that for C–C bonds it varies from 0.4 to 2.2 kcal/mol. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The quadratic configuration interaction calculation in the Gaussian-2 second-order Mo\ller–Plesset perturbation theory approach, G2(MP2), is replaced by a coupled-cluster (CC) singles and doubles calculation including a perturbational estimate of the triples excitations. In addition, the self-consistent-field (SCF) and MP2 geometry optimizations and SCF frequency calculation in the G2(MP2) approach are replaced by a density functional theory geometry optimization and frequency calculation [using the Becke three parameter hybrid functional with the Lee–Yang–Parr non-local correlation functional (B3LYP)] in the proposed G2(B3LYP/MP2/CC) approach. This simplification does not affect the average absolute deviation from experiment, but decreases the maximum error compared with the G2(MP2) approach. The G2(B3LYP/MP2/CC) atomization energies are compared with those obtained using the B3LYP approach, and the G2(B3LYP/MP2/CC) model is found to be more reliable, even if the B3LYP calculations are performed using a large basis set. [ABSTRACT FROM AUTHOR]

The dissociation energies of CN and C2 are determined with multireference configuration- interaction calculations utilizing very large Gaussian basis sets. Valence level one-particle errors, such as basis set incompleteness and basis set superposition error, are essentially eliminated by extrapolation. The errors in the correlation, or n particle, treatment are considered in detail. The effect of 1s correlation is computed using the averaged coupled-pair functional approach. Our best estimate for C2 is in very good agreement with the recent experimental result by Urdahl et al. Our best D0 value for CN is 7.72±0.04 eV. This is larger than previous theoretical values and in good agreement with the recent experimental result (7.738±0.02 eV) of Huang et al. The origin of the differences between the current D0 and our previous theoretical value is discussed. [ABSTRACT FROM AUTHOR]

The effect of N 1s correlation on the spectroscopic constants of N2 is studied using the coupled-cluster singles and doubles approach with a perturbational estimate of the connected triples [CCSD(T)] and internally contracted multireference configuration interaction (ICMRCI) techniques. At the ICMRCI level, we obtain a 1s effect of +1.35 kcal/mol on the dissociation energy. However, the effect is found to be smaller when size-extensive methods are used. The 1s effects computed at the CCSD(T) and internally contracted averaged coupled-pair-functional (ICACPF) levels are in excellent agreement. Our best estimate for the effect of 1s correlation is +0.8 kcal/mol on De, +9 cm-1 on ωe, and -0.002 Å on the bond length. Including our estimate for the effect of 1s correlation, we obtain a De of 227.1 using a correlation-consistent polarized-valence sextuple zeta basis set at the ICMRCI+Q level (where the +Q indicates that the Davidson correction has been included). Basis set incompleteness, which is estimated to be 0.7±0.2 kcal/mol, is still the major source of error. The CCSD(T) ten-electron results are found to be in excellent agreement with those obtained at the ICACPF or ICMRCI+Q levels of theory. [ABSTRACT FROM AUTHOR]

Potential energy curves for HeH, NeH, and ArH are determined using large basis sets and high levels of theory. The computed potential energy curves are in excellent agreement with the experimentally determined model potentials for HeH and NeH, but differ with the model potentials deduced for ArH by Tang and Toennies. We derive a potential based on the ab initio calculations and our best estimate of the well depth; using this potential we are able to reproduce the results of the orbiting resonance experiments of Toennies and co-workers. [ABSTRACT FROM AUTHOR]

Ab initio calculations employing large basis sets are performed to determine an accurate potential energy surface for H–H2 interactions for a broad range of separation distances. At large distances, the spherically averaged potential determined from the calculated energies agrees well with the corresponding results determined from dispersion coefficients; the van der Waals well depth is predicted to be 75±3μEh. Large basis sets have also been applied to reexamine the accuracy of theoretical repulsive potential energy surfaces (25–70 kcal/mol above the H–H2 asymptote) at small interatomic separations; the Boothroyd, Keogh, Martin, and Peterson (BKMP) potential energy surface is found to agree with results of the present calculations to within the expected uncertainty (±1 kcal/mol) of the fit. Multipolar expansions of the computed H–H2 potential energy surface are reported for four internuclear separation distances (1.2, 1.401, 1.449, and 1.7a0) of the hydrogen molecule. The differential elastic scattering cross section calculated from the present results is compared with the measurements from a crossed beam experiment. [ABSTRACT FROM AUTHOR]

Theoretical calculations have been performed to determine the spectroscopic constants for the ground and selected low-lying electronic states of the transition-metal noble-gas ions VAr+, FeAr+, CoAr+, CuHe+, CuAr+, and CuKr+. Analogous calculations have been performed for the ground states of the alkali noble-gas ions LiAr+, LiKr+, NaAr+, and KAr+ and the alkaline-earth noble-gas ion MgAr+ to contrast the difference in binding energies between the simple and transition-metal noble-gas ions. The binding energies increase with increasing polarizability of the noble-gas ions, as expected for a charge-induced dipole bonding mechanism. We find that the spectroscopic constants of the X 1Σ+ states of the alkali noble-gas ions are well described at the self-consistent field level. In contrast, the binding energies of the transition-metal noble-gas ions are substantially increased by electron correlation. The difference arises from the contribution of metal-neutral noble-gas-ion character in the wave functions. This correlation effect increases as the ionization potential of the noble gas decreases from He to Kr and as the ionization potential of the metal atom increases. [ABSTRACT FROM AUTHOR]

Energy-optimized, near Hartree–Fock quality GTO basis sets are reported for the first-row (Li to Ne) and third-row (K to Kr) atoms. The most accurate basis sets reported for the first row are (18s13p) sets which are within 4 μEH of the numerical Hartree–Fock (NHF) results. For B to Ne basis sets with more than 15s functions are quadruple zeta in the valence space. For the first-row transition metal atoms the (20s12p9d) basis sets are triple zeta in the valence space and are approximately equivalent to Clementi and Roetti’s accurate STO sets. Supplementing the (20s12p9d) basis sets optimized for the lowest state with the 4s23dn occupation with a diffuse d function gives SCF energy separations to the 4s13dn+1 and 3dn+2 states which are within 100 μEH of the NHF results. The most accurate basis sets for the transition metal atoms are within 30 μEH of the NHF results. In addition, energy optimized sets are reported for He(3P), Li(2P), and Be(3P). [ABSTRACT FROM AUTHOR]

Ab initio calculations are used to determine accurate spectroscopic constants (re, ωe, De ) for the X 2Π and A 2Σ+ states of the alkali sulfides (LiS–RbS) and the X 1Σ+, a 3Π, and A 1Π states of the alkaline–earth sulfides (BeS–SrS). In contrast to the alkali oxides, all of the alkali sulfides have X 2Π ground states owing to the larger electrostatic interaction. For the X 1Σ+ states of the alkaline–earth sulfides, our recommended dissociation energies (D0 in eV) are BeS(3.27), MgS(2.32), CaS(3.29), and SrS(3.41), in overall good agreement with the (uncertain) experimental values. Our computed Te(a 3Π) and Te(A 1Π) values for BeS of 7269 and 8244 cm-1 are in excellent agreement with experiment. For the heavier alkaline–earth sulfides, our calculated Te(a 3Π) values (cm-1) are MgS(4218), CaS(6528), and SrS(6870), and our calculated Te(A 1Π) values are MgS(5068), CaS(6901), and SrS(7179). Core correlation increases the Te values for the a 3Π and A 1Π states of MgS, CaS, and SrS. [ABSTRACT FROM AUTHOR]

Theoretical spectroscopic constants are reported for the A’ 5Σ+g and C‘ 5Πu states of N2 based on CASSCF/MRCI calculations employing large ANO Gaussian basis sets. Our calculated A’ 5Σ+g potential differs qualitatively from previous calculations in that the inner well is significantly deeper (De ≊3450 cm-1). The potential also has a substantial barrier (≊500 cm-1) to dissociation with a maximum near 4.3 a0, and then a shallow (≊47 cm-1) van der Waals minimum near 6.3 a0. We suggest that it is the v=4 and v=5 levels that are involved in predissociating the a 1Πg and B 3Πg states, as opposed to v=0 and v=1 proposed by Verma. The deeper well in the A’ 5Σ+g state provides considerable support for the theory of Berkowitz, Chupka, and Kistiakowsky who proposed that A’ 5Σ+g is the primary precursor state involved in the yellow Lewis–Rayleigh afterglow of nitrogen. The theoretical De for the C‘ 5Πu state is also much larger than previous estimates. The previously unassigned Hermann infrared system (HIR) band positions correspond exceptionally well to our theoretical values for the C‘ 5Πu →A’ 5Σ+g transition. This and other considerations lead to a rather convincing assignment of this transition as the HIR system. Einstein coefficients and radiative lifetimes are presented for this transition. Since the natural radiative lifetime of this state is much longer than experimental estimates, the C‘ 5Πu state may derive its actual lifetime through interaction with the C 3Πu state. [ABSTRACT FROM AUTHOR]

Energy optimized, near Hartree–Fock quality Gaussian basis sets ranging in size from (17s12p) to (20s15p) are reported for the ground states of the second-row atoms and for Na (2P), Na+, Na-, Mg (3P), P-, S-, and Cl-. In addition, optimized supplementary functions are given for the ground state basis sets to describe the negative ions, and the excited Na (2P) and Mg (3P) atomic states. The ratios of successive orbital exponents describing the inner part of the 1s and 2p orbitals are found to be nearly independent of both nuclear charge and basis set size. This provides a method of obtaining good starting guesses for other basis set optimizations. [ABSTRACT FROM AUTHOR]

The spectroscopic parameters and separations between the three low-lying X 3Πu, A 3Σ-g, and a 1Σ+g states of Al2 are studied as a function of both the one-particle and n-particle basis sets. Approximate correlation treatments are calibrated against full CI calculations correlating the six valence electrons in a double-zeta plus two d-function basis set. Since the CASSCF/MRCI 3Πu–3Σ-g separation is in excellent agreement with the FCI value, the MRCI calculations were carried out in an extended (20s13p6d4f)/[6s5p3d2f] Gaussian basis. Including a small correction for relativistic effects, our best estimate is that the 3Σ-g state lies 174 cm-1 above the 3Πu ground state. The 1Σ+g state lies at least 2000 cm-1 higher in energy. At the CPF level, inclusion of 2s and 2p correlation has little effect on De, reduces Te by only 26 cm-1, and shortens the bond lengths by about 0.02 a0. Further strong support for a 3Πu ground state comes from the experimental absorption spectra, since both observed transitions can be convincingly assigned as 3Πu→3Πg. The (2)3Πg state is observed to be sensitive to the level of correlation treatment, and to have its minimum shifted to shorter r values, such that the strongest experimental absorption peak probably corresponds to the 0→2 transition. [ABSTRACT FROM AUTHOR]

Examines the role of bond functions in the calculation of accurate bond energies. Computation bound diatomics bond energy using basis sets; Extrapolation of the results obtained with basis sets; Impact of bond function addition in a valence basis set on bond energies.

Focuses on the determination of a high quality ab initio potential energy surface (PES) and dipole moment function for water. Empirical adjustment of the PES to improve the agreement between the computed line positions; Small changes in the PES including an estaimate of core electron correlation which greatly improves the agreement with the experiment.

Theoretical potentials for the 7Σ+u state are reported using both extended Slater and Gaussian basis sets. Electron correlation is included using the interacting correlated fragments (ICF), the singles plus doubles configuration-interaction (SDCI), and the coupled-pair functional (CPF) approaches. Our best potential, corrected for basis set superposition errors, has a well depth of about 21 cm-1 and an re of 7.52 bohr. The inclusion of the nitrogen 2s correlation significantly increases the well depth. These results are in reasonably good agreement with the empirical potential of Ferrante and Stwalley, and support the contention that spin-polarized atomic nitrogen should behave like a classical solid such as Ne. It is shown that the modified Buckingham potential used by Ferrante and Stwalley is consistent with our ab initio potentials if smaller values are used for the dispersion coefficients of nitrogen atom. [ABSTRACT FROM AUTHOR]

Chemisorption of oxygen and hydrogen atoms onto a 13 atom aluminum cluster is investigated theoretically. Oxygen atoms are found to remain on the outside of the cluster, and at low coverage, to expand the Al–Al bond length by 0.42 bohr. Hydrogen atoms penetrate into the cluster resulting in a sizeable, 0.57 bohr expansion at high coverage. The difference between chemisorption on aluminum and beryllium clusters is discussed. [ABSTRACT FROM AUTHOR]

Theoretical spectroscopic parameters are presented at the self-consistent-field and singles-plus-doubles configuration-interaction (SDCI) levels for the ground states of the positive ions of the alkaline-earth fluorides, oxides, and hydroxides. The alkaline-earth fluoride and hydroxide ions are found to have X 1Σ+ ground states corresponding to a M+2X-1 structure. The alkaline-earth oxide positive ions, on the other hand, undergo a change in ground state symmetry between CaO+(X 2Π) and SrO+(X 2Σ+), whereas the analogous alkali oxides undergo the change one row higher between NaO and KO. The greater stability of the 2Π states for the alkaline-earth oxide positive ions is due to the increased electrostaticfield created by the larger metal ion charge and reduced bond lengths. Calculations on the 3Π states of BeF+, MgF+, BeOH+, and MgOH+ indicate that these states, corresponding to the M+1X structure, lie much higher in energy. The bond distances of the positive ions are uniformly 0.07±0.02 Å shorter than the analogous electronic states of the neutral. By correcting for the relatively small errors in the theoretical bond distances for the neutrals, bond distances accurate to about 0.005 Å are obtained for the ions where experimental values are available for the neutral species. Since extensive one-particle basis sets are employed and differential correlation effects are minimized by dissociating directly to ions, theoretical dissociation energies and ionization potentials (I.P.) accurate to about 0.15 eV can be obtained. The dissociation energies (D0) in eV for the ground states at the SDCI level are found to be: BeO+(4.04), MgO+(2.31), CaO+(3.29), SrO+(3.28), BeF+(6.25), MgF+(4.46), CaF+(5.67), SrF+(5.71), BeOH+(6.00), MgOH+(3.62), CaOH+(4.73), and SrOH+(4.67). For all of the systems studied, the bonding is dominated by electrostatic interactions. However, the molecules containing Be show some covalent character owing to... [ABSTRACT FROM AUTHOR]

The structure and binding energies are determined for many of the M(H2O)+n and M(H2O)2+n species, for n=1–3 and M=Mg, Ca, or Sr. The trends are explained in terms of metal sp or sdσ hybridization and core polarization. The M(NH3)+n systems, with M=Mg or Sr, are also studied. For the positive ions, the low-lying excited states are also studied and compared with experiment. The calculations suggest an alternative interpretation of the SrNH+3 spectrum. [ABSTRACT FROM AUTHOR]

The successive H2O binding energies of Na(H2O)+n are in excellent agreement with experiment and are rather insensitive to electron correlation since the bonding is predominantly electrostatic. A point-charge model shows that changes in the successive binding energies are due primarily to ligand–ligand repulsion. Vibrational frequencies and infrared intensities are determined for Na(H2O)+n (n=1–4) at the self-consistent-field and second-order Mo\ller–Plesset levels of theory to facilitate experimental study of these ions. [ABSTRACT FROM AUTHOR]

Ab initio calculations including high levels of correlation treatment have been performed for the B 2Π–X 2Π β system of NO. The calculations are complicated by the fact that the B 2Π state gains considerable Rydberg character in the Franck–Condon region of this transition, as it approaches an avoided crossing with the C 2Π Rydberg state at shorter r values. The calculated lifetimes vary from 2.12 to 1.17 μs for v’=0 to 6, as compared with the recent measurements using laser induced fluorescence that vary from 2.00 to 0.85 μs. Nearly exact agreement with the experimental lifetimes can be achieved by shifting the transition moment function to larger r values to simulate a more rapid onset of the Rydbergization of the B 2Π state. The calculated Einstein coefficients for v’=6 to v‘=0–16 are in overall good agreement with experiment, but systematic differences remain that may be partially attributable to limitations in the calibration of the spectrometer. [ABSTRACT FROM AUTHOR]

The successive binding energies of up to four H2O and NH3 ligands to Cu+ are computed at the self-consistent-field and modified coupled-pair functional levels. The most stable structures are those where all ligands are equivalent. Replacing Cu+ by a point charge gives binding energies that are in good agreement with ab initio and experimental results, and is consistent with bonding that is largely charge dipole in nature. About two-thirds of the large reduction in ligand binding energy between the second and third ligand is due to ligand–ligand repulsion, while one-third is due to increased metal–ligand repulsion resulting from a loss of sdσ hybridization. The first and second ligand binding energies increase substantially at the correlated level due to an improved description of sdσ hybridization. [ABSTRACT FROM AUTHOR]

The metal–carbon bond dissociation energies (D0) and geometries for the first- and second-row transition-metal methyl neutrals and positive ions are determined. The computed D0 values for the positive ions compare favorably with experiment, except for RuCH+3, RhCH+3, and PdCH+3 where the experimental values are 10–15 kcal/mol larger. The computed D0 values for the hydride and methyl positive ions are similar for all metals in both transition rows except for Cu and Ag. However, for the neutral systems the D0 values for the methyls are smaller, especially on the right-hand side of both transition rows where the differences approach 15 kcal/mol. In general, the dissociation energies do not follow simple trends, as the individual D0 values are significantly affected by the relative spacings between the atomic states of the metal. The study of all of the methyl neutral and ions of both transition rows presented here provides a consistent set of data for the dissociation energies, thereby allowing a critical assessment of the experimental data for these molecular species, and an enhanced understanding of the different bonding mechanisms. [ABSTRACT FROM AUTHOR]