Showing papers on "Ab initio quantum chemistry methods published in 1968"

Ab Initio Calculations on 62 Low‐Lying States of the O2 Molecule

Abstract: Ab initio calculations have been made on the 62 low‐lying states of molecular O2 which result from the combination of O atoms in 3P, 1D, and 1S atomic states. The calculations are done at nine different internuclear separations, and potential‐energy curves are presented for all states. Twelve bound states were found: the lowest seven have been observed; two others have been predicted before; three are new. The state ordering agrees with experiment except for the c 1Σu− state. Possible reasons for this discrepancy are discussed. The remaining errors in the bound‐state energy separations are rationalized. Data possibly bearing on the unobserved bound states are cited. Repulsive‐state curves are used to discuss predissociation in the Schumann–Runge bands and to illustrate avoided‐crossing phenomena.

Ab initio calculations on small hydrides including electron correlation: I. The BeH2 molecule in its ground state

Abstract: A method of calculating directly approximate natural orbitals in the electron pair approximation is applied to the ground state of the BeH2 molecule, which is not known from experiment. The question of optimum localization of the electron pairs for asymmetric nuclear configuration receives particular attention. The interpair correlation energy is estimated. The physical properties of the molecule are predicted and experimental conditions are indicated under which it should be observed.ZusammenfassungDie Methode zur direkten Berechnung genäherter natürlicher Orbitale in der Elektronenpaar-näherung wird auf den Grundzustand des vom Experiment her unbekannten BeH2-Moleküls angewandt. Die Frage der optimalen Lokalisierung der Elektronenpaare für asymmetrische Anordnung der Kerne findet besonderes Interesse. Die Interpaar-Korrelationsenergie wird abgeschätzt. Die vorausgesagten physikalischen Eigenschaften dieses Moleküls sind in einer Tabelle zusammengestellt. Bedingungen, unter denen das Molekül experimentell beobachtet werden sollte, werden angegeben.RésuméLa méthode basée sur le calcul direct des orbitales naturelles approchées est appliquée á l'étude de l'état fondamental de la molécule BeH2 qui est inconnu du point de vue expérimental. La question de la localisation optimale des paires d'électrons pour des configurations asymmétriques des noyaux est discutée en détail. La corrélation interpaires est estimée. On prédit les propriétés physiques de la molécule et on indique les conditions qui devraient permettre de l'observer.

Theoretical Calculation of the Electronic States of C2

Abstract: Results of ab initio calculations are presented for a series of valence levels of C2+ and for the two lowest states of C2 (x 1Σg+and X′ 3Πu). Correlation energies of all the levels are estimated by a new scheme taking into account near‐degeneracy effects in the molecule and in the constituent ion pairs. Results for C2 are in close agreement with experiment for equilibrium internuclear distances, vibration frequencies, term energies and dissociation energies. For C2+ the calculations show the ground state to be 4Σg− with relatively low‐lying (∼ 0.6 eV) 2Δg and 2Πu levels. The electron‐impact measurement for the appearance potential of C2+ is interpreted as C2(3Πu, υ = 1) → C2+(4Σg−, υ = 0). The quartet transition 4Σg− → 4Σu− is predicted to lie at 6000 A. The major doublet transitions all lie in the infrared.

VESCF-MO studies of molecules containing atoms from the second row of the Periodic Table. II. Properties of the fluorides of silicon, phosphorus, sulphur, and chlorine for a minimal basis set excluding 3d-orbitals

Abstract: A study has been made of the electronic structures of the fluorides of silicon, phosphorus, sulphur, and chlorine by the VESCF molecular-orbital method with a minimal basis set, not including 3d-orbitals on the central atom. It proves possible to understand variations in bond lengths and charges on fluorine ligands, dipole moments, force constants, and some n.q.r. data. The calculations are found to be sensitive to assumptions about scaling factors for monocentric coulomb repulsion integrals and penetration integrals. We have not discovered any justification for including 3d-orbitals in the description of the electronic structure of these molecules.

Ab Initio Studies of Small Molecules Using 1s Gaussian Basis Functions. III. LCGTO SCF MO Wavefunctions of the Three‐ and Four‐Electron Systems He2+, He2, and Linear H3, H4+, H4

Abstract: The ground states of the three‐ and four‐electron systems He2+, He2, and linear symmetric H3, H4+, and H4 are investigated using the linear combinations of Gaussian‐type orbitals (LCGTO) self‐consistent‐field molecular‐orbital (SCF MO) approach The basis functions are 1s GTO's, not necessarily located at nuclear positions Optimized, near‐Hartree–Fock (HF) wavefunctions are presented for the isoelectronic H3 and He2+ systems, for which such results were not previously available They are found to have similar correlation energies With simpler wavefunctions, E‐vs‐R plots for H3 and He2+ give good bond lengths and shapes of the energy curve It is found that previous SCF MO studies have not closely approximated the true HF function for He2 The H4+ calculations are the first SCF MO results available for the species, and indicate its instability with respect to H3+ + H, in agreement with recent experimental data The GTO studies for H4 are the only rigorously executed ab initio calculations yet done for th

Method of Diatomics‐in‐Molecules. IX. Ground and Excited States of H4 and the H2,H2 Bimolecular Exchange Reaction

Abstract: The semiempirical diatomics‐in‐molecules theory is used to predict potential‐energy surfaces for the ground and several excited electronic states of the system H4 Results are presented for linear (D∞h), rectangular (D2h), square (D4h), “perpendicular” (D2d), tetrahedral (Td), and rhombic (D2h) conformations Results are compared with those from previous ab initio calculations The lowest energy state of square H4 is singlet; the optimum internuclear distance is R = 20 bohrs, for which the energy is 678 kcal/mole higher than 2E(H2) (fixed nuclei energies) Vibrational frequencies for this square are computed: 2180, 1210, 1990, 1470, 1470, and 2280i cm−1, the latter corresponding to the reaction coordinate for the H2, H2 bimolecular exchange reaction The specific rate constant for this reaction is computed according to the absolute rate theory

Quantum‐Statistical Ab Initio Calculations on Ionic Crystals

Abstract: A model of the ionic crystal is proposed on the basis of the atomic quantum‐statistical equation with exchange. The crystal is composed of ionic spheres touching one another. The equilibrium state of the crystal is the result of electrostatic attraction being balanced by repulsion due to the electron gas pressure acting on the surfaces of the ions. Several empirical, or semiempirical, quantities relating to the properties of the crystals could be estimated for the first time by an ab initio calculus. These were the univalent and multivalent radii of crystal ions, the compressibilities and cohesive energies of crystals containing the multivalent ions, and the energies of ultraviolet absorbtion maxima; also several structrual properties could be predicted from purely theoretical considerations. Only compounds whose ionic components had an atomic number not exceeding 40 have been examined.

Quantum Mechanics of the H2–H2 Interaction. V. The Importance of Intermolecular Charge‐Transfer States

Abstract: In a preceding paper a one‐configuration wavefunction of strong orthogonal group functions was found to give a rather poor description of the orientation dependence of short‐range intermolecular forces in H2–H2. As an alternative to the necessity of relaxing the strong orthogonality condition, it is suggested that the introduction of a small amount of the charge‐transfer state H2−–H2+ describing one‐electron transfer between the two molecules should improve the interaction energy and its change with the relative molecular orientation. Ab initio calculations in a region of intermolecular separations ranging from two to four times the internuclear equilibrium distance of an isolated H2 molecule show that less than 1% of such a charge‐transfer state gives interaction energies which are in substantial agreement with those obtained with a complete configurational interaction.

Calculation of the Electron Affinity of Boron

Abstract: Ab initio calculations have been made for the ground states of neutral B and ${\mathrm{B}}^{\ensuremath{-}}$ using configuration-interaction wave functions which included the Hartree-Fock configuration, single excitations, and double excitations of the types ($1s,2p$), ($2s,2s$), ($2s,2p$), and ($2p,2p$). These calculations indicate that ${\mathrm{B}}^{\ensuremath{-}}$ is stable and that the electron affinity of B is 0.0028 Hartrees (0.08 eV). Contributions from the omitted types of correlations were investigated by making separate-pair calculations for the ($1s,1s$) and ($1s,2s$) pairs. These contributions do not affect the qualitative conclusions. Consideration of the probable errors in the calculations leads to the definite conclusion that ${\mathrm{B}}^{\ensuremath{-}}$ is stable and to an estimate of 0.25 eV for the electron affinity of boron.

Perturbation theory for intermolecular forces : application to some adsorption models

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