Stephen V. O'Neil

INTERACTION POTENTIAL BETWEEN TWO RIGID HF MOLECULES

As a prelude to the study of energy transfer in the HF–HF system, the potential energy surface for the interaction of two rigid HF molecules has been calculated within the ab initio self‐consistent‐field framework. An H(4s 1p/2s 1p), F(9s 5p 1d/4s 2p 1d) basis set of contracted Gaussian function was employed. The number of unique points on the surface is greatly reduced by symmetry, and only 294 points were required to give a fairly complete description of the four‐dimensional surface. Parts of the surface are illustrated by a series of contour maps. Some preliminary attempts to fit the surface to an analytic form are described. The equilibrium geometry of (HF)2 is predicted.

Potential Energy Surface Including Electron Correlation for the Chemical F + H2 → FH + H I. Preliminary Surface

Rigorous quantum mechanical calculations have been carried out for about 150 linear and 200 non‐linear geometries for the FH2 system. The contracted Gaussian basis set used consisted of four s and two p functions on fluorine and two s functions on hydrogen. The barrier height and exothermicity are poorly predicted by single configuration self‐consistent‐field calculations. However, the 214‐configuration correlated results are in qualitative agreement with experiment (low barrier height and substantial exothermicity). The reaction coordinate is discussed, and pictures of the potential surface are presented. A second series of calculations is being carried out with a larger basis set. These latter calculations yield nearly quantitative agreement with experiment for both the barrier height and exothermicity.

Valence‐Excited States of Carbon Monoxide

Precise quantum mechanical calculations have been performed on the 72 states of carbon monoxide which dissociate to a 3P, 1D, 1S, or 5S carbon atom plus a 3P, 1D, or 1S oxygen atom. A minimal basis set of Slater‐type orbitals (optimized for the C and O atoms) was used, and holding the atomic 1s orbitals doubly occupied, full configuration interaction calculations were carried out for all molecular states at no fewer than nine internuclear separations. Seventeen bound states (De ≥ 0.27 eV) were obtained, eight of which have been observed experimentally. The theoretical ordering of known bound states agrees with experiment except for the a 3Π and A 1Π states. This fact is rationalized by noting that these two states have much smaller re values than do the other excited states. The most interesting of the unobserved predicted bound states are the 5Σ+ and 5Π states, which dissociate to 3PC+3PO, and the third 1Π state, with a relatively large calculated dissociation energy. A dominant molecular orbital configu...

Avoided intersection of potential energy surfaces: The (H+ + H2, H + H2+) system

Nonempirical electronic structure calculations have been carried out on the two lowest 1A1 states of H 3+. When one proton is infinitely separated from the other two, these 1A1 potential surfaces cross each other. The nature of this avoided intersection is examined by means of potential curves, contour diagrams, and perspective plots. Surface hopping is discussed within a Landau‐Zener‐Stuckelberg (LZS) framework and the LZS assumptions concerning the surfaces are shown to be reasonable near the avoided intersection. Ab initio LZS parameters are compared with those obtained from the semiempirical diatomics‐in‐molecules surfaces of Preston and Tully. The agreement is good, better than might have been anticipated.

C2υ Potential Energy Surfaces for Seven Low‐Lying States of CH2

Ab initio calculations have been carried out at 28 C2υ geometries for the lowest 1A1, 1A2, 3A2, 1B1, 3B1, 1B2, and 3B2 states of CH2. The basis set used was of the contracted Gaussian type with four s and two p functions on carbon and two s functions on hydrogen. In all calculations except 1A1 the SCF configuration plus all singly and doubly excited configurations were included (holding the K shell frozen), and the iterative natural orbital procedure was used to obtain an optimum set of orbitals. For the 1A1 state a two configuration SCF calculation was used as the starting point for the configuration interaction calculations. In a preliminary communication we predicted the triplet ground state of CH2 to be bent, and this prediction has since been justified experimentally by Bernheim et al. and by Wasserman et al. For the 1A1 state the ab initio geometry is r = 1.13 A, θ = 104°, compared to experiment, r = 1.11 A, θ = 102°. For the 1B1 state the predicted geometry is r = 1.09 A, θ = 144°, as opposed to ex...

Geometries of the excited electronic states of HCN

Ab initio quantum mechanical electronic structure calculations have been carried out for the ground state and 12 low‐lying (< 10 eV) excited states of HCN. A contracted Gaussian basis set of essentially double zeta quality was employed. A new theoretical approach, which should be widely applicable, was applied to the excited electronic states. First one selects a physically meaningful set of orbitals, which, hopefully, will be about equally suitable for all the electronic states of interest. After selecting a single configuration to describe each electronic state, configuration interaction is performed including all configurations differing by one orbital from any of the selected reference configurations. The method appears to be one of the simplest capable of treating several states of the same symmetry. The predicted geometries have been compared with the experimental results of Herzberg and Innes, as well as the appropriate Walsh diagram. The ab initio calculations and the Walsh diagram concur that Her...

On the H+F2→HF+F reaction. An ab initio potential energy surface

Rigorous quantum mechanical calculations have been carried out to predict the H+F2→HF+F potential energy surface. A double zeta basis set was used, and open‐shell self‐consistent‐field (SCF) calculations were carried out. In addition, electron correlation was explicitly treated using first‐order wavefunctions, made up of 555 2A′ configurations. Orbitals were optimized by the interative natural orbital method. From the SCF calculations the barrier height and exothermicity are predicted to be 12.2 and 132.4 kcal/mole, respectively. The configuration interaction (CI) values are 1.0 and 88.3 kcal, in much better agreement with the experimental values, 1.2 and 102.5 kcal. The saddle point is predicted from the CI calculations to occur for a linear geometry, R(H–F)=2.05 A, R(F–F)=1.57 A. This corresponds to an H–F separation more than twice as great as in the HF molecule but an F–F separation is only slightly (0.03 A) longer than in the isolated F2 molecule. A substantial number of calculations were carried out...

Role of Electron Correlation in a Priori Predictions of the Electronic Ground State of BeO

Ab initio wavefunctions including electron correlation have been calculated for the 3II state of BeO. A (4s2p1d) basis set of Slater functions was centered on each atom. The iterative natural orbital method was used to optimize the set of molecular orbitals employed in each 591 configuration first‐order wave‐function. The 3II energy calculated here is 0.73 eV above the 1Σ+ energy obtained in a comparable calculation. Since near Hartree‐Fock calculations result in a 3II energy below the 1Σ+ energy, it seems clear that electron correlation plays a crucial role in the ordering of these states. Predicted spectroscopic constants for the 3II state are: Re=1.463 A, ωe= 1270 cm−1, and Be= 1.365 cm−1. Natural orbital occupation numbers and coefficients of important configurations in the CI wavefunctions are presented to describe the electronic structure of 3II BeO. First‐order calculations (519 configurations) were also carried out for the lowest 3Σ− state of BeO. These calculations confirm our previous SCF predic...

Configuration Interaction Study of the X 3Σ−, a 1Δ, and b 1Σ+ States of NH

Using a (3s, 2p, 1d / 2s, 1p) basis set of contracted Slater‐type functions and an iterative natural orbital scheme, ab initio valence configuration interaction studies have been done on the lowest three states of the imidogen radical at eight internuclear separations. Included in the CI were those configurations differing by zero, one, or two space orbitals from the Hartree–Fock configuration, except that the 1σ orbital was held doubly occupied. The size of the CI varied from 259 (1Δ) to 418 (3Σ−). For the ground state the computed total energy lies below that reported in any previous calculation except the 3379 configuration wavefunction of Bender and Davidson. From the potential curves thus obtained, the spectroscopic constants re, ωe, ωexe, Be, and αe are calculated, and compare well with the available experimental constants. The molecular splittings are calculated to be 2.00 eV (X 3Σ− − a 1Δ) and 0.79 (a 1Δ − b 1Σ+), but when the discrepancy between calculated and experimental atomic limits is taken ...

Repulsive 3∑− and low-lying (⩾ 1.9 eV) 3∑+ states of BeO

Abstract Ab initio calculations have been carried out on the lowest 3 ∑ − and 3 ∑ + states of beryllium oxide. A “double zeta plus polarization” set of Slater functions was used. The self-consistent-field wavefunction for the 3 ∑ − state dissociates properly to ground state Be and O atoms and is repulsive. Electron correlation was explicitly considered for the 3 ∑ + state using “first-order” wavefunctions, which have yielded reliable dissociation energies for other diatomic molecules. The 3 ∑ + state, which has not been observed experimentally, is predicted to lie 1.91 eV above the lowest 1 ∑ + state. Predicted spectroscopic constants for the 3 ∑ + state are r e = 1.384 A, ω e = 1234 cm −1 , and B e = 1.527 cm −1 .