Ab initio molecular orbital calculations

A series of non-empirical calculations on furan, pyrrole and 1,2,5-oxadiazole are reported in which the effect of polarisation functions added to the minimal 7s 3p basis on each atom is studied. The effect on these planar molecules is largely through the rather than the-system. A comparison with the results of work with scaled functions is reported. Both series are shown to lead to much improved agreement with the electron spectroscopy energy levels. The effect on the dipole moments of these changes in basis is more variable but, with the exception of furan, the agreement with experiment is improved in the present method.

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Zusammenfassung Für die Moleküle Furan, Pyrrol und 1,2,5-Oxadiazol wurde eine Reihe von nichtempirischen Rechnungen durchgeführt, in denen der Einfluß von zusätzlichen Polarisationsfunktionen zur minimalen 7s 3p-Basis an jedem Atom untersucht wird. Die Ergebnisse werden mehr durch die Art der Beschreibung des Systems der-Elektronen als durch diejenige der-Elektronen beeinflußt. Ein Vergleich mit den Ergebnissen bei Verwendung skalierter Funktionen wird durchgeführt. Beide Reihen von Ergebnissen zeigen eine verbesserte Übereinstimmung zu den Energiemeßwerten der Elektronenspektroskopie. Die Änderungen des berechneten Dipolmoments bei derartigen Basisvariationen sind größer als bei früheren Methoden. Die Übereinstimmung mit dem Experiment wird, mit Ausnahme von Furan, jedoch verbessert.

     

Ab initio molecular orbital calculations on the pyrazine-lithium ion pair

The results of ab initio molecular orbital calculations are presented for the pyrazine^?/Li⁺ ion pair. It is shown that the lowest energy conformation is where the lithium occupies a position in the plane of the pyrazine ring, along the C ₂ axis passing through the nitrogen atoms.     

Ab initio molecular orbital calculations for butadiene and its ions

Ab initio calculations are reported for cis and trans butadiene and some of their ions. The calculations are compared with semi-empirical results, and used to predict coupling constants.     

Ab initio calculations on small hydrides including electron correlation

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.

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Zusammenfassung Die 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.

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Résumé La 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.

     

Ab initio calculations on small hydrides including Electron correlation

The molecules BeH₂, Be₂H₄ and Be₃H₆ are investigated by means of ab initio calculations including the electron correlation of the valence shell electrons. It is found that BeH₂ shows a strong tendency to polymerize in linear chains. The polymerization energy is estimated to be 40 Kcal/Mol.

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Zusammenfassung Die Moleküle BeH₂, Be₂H₄ und Be₃H₆ werden mit Hilfe von ab initio Rechnungen unter Einschlu\ der Elektronenkorrelation der Valenzelektronen untersucht. Es zeigt sich, da\ BeH₂ eine starke Tendenz hat, in linearen Ketten zu polymerisieren. Die Polymerisationsenergie wird zu 40 Kcal/Mol abgeschÄtzt.

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Résumé Les molécules BeH₂, Be₂H₄ et Be₃H₆ sont étudiées au moyen de calculs ab-initio avec corrélation électronique des électrons de la couche de valence. On trouve que BeH₂ présente une forte tendance à polymériser en chaÎnes linéaires. L'énergie de polymérisation est estimée à 40 Kcal/mole.

     

Ab initio calculations on small hydrides including electron correlation

The two lowest electronic states (³ B ₁ and ¹ A ₁) of the methylene radical (CH₂) are calculated both in SCF-approximation and with the IEPA-PNO method (including electron correlation). The influence of polarization functions and electronic correlation on the shape of the potential curves of the two states is discussed. The calculated equilibrium geometries agree very well with experiment, but the results for transition energies (e.g. ³ B ₁¹ A ₁ excitation energy=9.2 kcal/mole, total binding energy=187 kcal/mole) are more reliable than the existent experimental values.     

Ab initio calculations of small hydrides including electron correlation

Calculations based on the independent-electron-pair approximation (IEPA) and the direct determination of approximate natural orbitals for the different pair correlation functions, including intra- and interpair correlation effects, are performed for the BH ground state at several internuclear distances r. The dependence of the different pair correlation contributions on r is investigated. The contributions involving the K-shell orbitals of B are practically independent of r. Calculated equilibrium distances and force constants including correlation effects are in better agreement with experiment than are the corresponding Hartree-Fock values.

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Zusammenfassung Rechnungen, die auf der Näherung der unabhängigen Elektronenpaare (Independent-electron-pair approximation IEPA) und der direkten Bestimmung genäherter natürlicher Orbitale beruhen und die sowohl Intra- als auch Interpaar-Korrelationseffekte erfassen, wurden für das BH-Molekül bei verschiedenen interatomaren Abständen r durchgeführt. Die Abhängigkeit der verschiedenen Paar-Korrelationsbeiträge von r wird untersucht. Die Beiträge, an denen die K-Schale des B-Atoms beteiligt ist, erweisen sich als praktisch unabhängig von r. Die Berücksichtigung der Korrelationseffekte führt zu einer Verbesserung der berechneten Werte von Gleichgewichtsabstand und Kraftkonstante in Richtung auf die experimentellen Werte.

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Résumé Un calcul basé sur l'approximation des paires d'électrons indépendantes (Independent-electron-pair approximation IEPA) et la détermination directe des orbitales naturelles approchées a été fait pour l'état fondamental de la molécule de BH; les contributions provenant de la corrélation intra- et interpaire ont été calculées pour plusieurs distances internucléaires r, et la dépendance de ces contributions en fonction de r a été étudiée.On montre que les contributions dans lesquelles le niveau K intervient sont pratiquement independantes de r. Le fait de tenir compte explicitement de la corrélation améliore les valeurs calculées de la distance d'équilibre et de la constante de force qui se rapprochent des valeurs expérimentales.

     

Ab initio calculations of small hydrides including electron correlation

Five different structures of CH₅ ⁺ and one structure of CH₅ ^– are calculated using a gaussian basis both in the SCF approximation and with the inclusion of electron correlation in the independent electron pair approximation (IEPA). While on SCF level the C _sstructure of CH₅ ⁺ has to lowest energy, the energy difference between the C _sand C _2vstructures becomes negligible if correlation is included. In contrast to this the approach of a proton to CH₄ at large and intermediate distances is most favorable towards a corner of the CH₄ tetrahedron which means a structure. The decomposition of CH₅ ⁺ into CH₃ ⁺ and H₂ requires 20kcal/mol on SCF level and 40 kcal/mol if correlation is included.     

Ab Initio molecular orbital calculations

Linear combinations of Gaussian orbitals were contracted to minimal basis sets. The binding energies (B.E.) obtained for the three compounds furan, pyrrole and 1,2,5-oxadiazole are 0.75, 0.84 and 0.26 a.u. respectively. Correlation of these figures and those obtained in related calculations leads to the relationship (B.E.)_exp.=0.7347 (B.E.)_calc.+633 (Kcal/mole). The photoelectron spectra of these compounds are extended and correlated with the orbital energies; the -electron excitations are given by (I.P.)_exp.=1.00 (I.P.)_calc.–2.20 eV. Mulliken population analyses give dipole moments in reasonable agreement with experiment.

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Zusammenfassung Linearkombinationen von Gauß-Orbitalen wurden zu minimalen Basissätzen kontrahiert.Die Bindungsenergien (B.E.) für die drei Komponenten Furan, Pyrrol und 1,2,5-Oxadiazol haben die Werte: 0,75, 0,84 und 0,26 a.u. Setzt man diese Werte zu den berechneten in Beziehung, so erhält man die Gleichung: (B.E.)_exp.=0,7347 (B.E.)_calc.+633 (Kcal/Mol).Die Photoelektronenspektren dieser drei Komponenten werden erweitert und mit den Orbitalenergien korreliert; die -Elektronen-Anregungsenergien werden durch die Beziehung: (I.P.)_exp. =1.00 (I.P.)_calc.–2,20 eV gegeben. Mulliken-Populations-Analysen ergeben die Dipolmomente in bemerkenswerter Übereinstimmung mit experimentellen Werten.

     

Ab initio molecular orbital calculations on the water-carbon dioxide system: Molecular complexes

An ab initio LCAO MO SCF calculation has been carried out on three different complexes between water and carbon dioxide. It has been found that a planar complex in which the carbon atom is bound to the oxygen atom in the water is the most stable, with a calculated energy of formation of ?25.7 kJ/mol.     

Ab initio calculations of small hydrides including electron correlation

The harmonic force constants and vibration frequencies of BH₃ (which is, not directly accessible to experimental studies) are calculated both in SCF approximation and including correlation in the IEPA-PNO scheme, using a Gaussian basis set. The results are compared with those of related molecules like BH and BH₂.

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Zusammenfassung Die harmonischen Kraftkonstanten und Schwingungsfrequenzen des BH₃ (das experimentellen Untersuchungen nicht unmittelbar zugänglich ist) werden sowohl in der SCF-Näherung als auch unter Einschluß der Elektronenkorrelation in der IEPA-PNO-Näherung mit einer Basis von Gaußfunktionen berechnet. Die Ergebnisse werden mit denen verwandter Moleküle wie BH und BH₂ verglichen.

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Résumé Les constantes de force et frequences de vibration harmoniques de la molecule BH₃ (qui est inaccessible a des etudes experimentales directes) sont calculées dans le cadre des methodes SCF et IEPA-PNO utilisant une base d'orbitales Gaussiennes. On compare les resultats avec ceux obtenus recemment pour des molecules comme BH et BH₂.

     

Ab initio molecular orbital calculations on beryllium and magnesium atom reactions with water

Ab initio molecular orbital calculations are reported for beryllium and magnesium atom complexes with water, Be. OH₂ and Mg OH₂, and for species resulting from insertion of the metal atom(s) into the water molecule, HBeOH, HMgOH, HBeOBeH, and HMgOMgH     

Ab initio and electron propagator theory study of boron hydrides

Boron hydrides (BH3, B2H6, B3H7, B4H10, B5H9, and B5H11) and their cations are studied by the coupled cluster CCSD(T) theory, the second-order Mller-Plesset (MP2) perturbation method, and the electron propagator theory in the partial third-order quasi-particle approximation, using the 6-311G(d,p) basis set. The vertical ionization potential energies are calculated, indicating an excellent agreement with the experimental data from photoelectron spectroscopy. Assignments to the experimental spectra are made on the basis of the present computational analyses. A significant Jahn-Teller effect on BH3+ leads to two states, 2A1 and 2B2, with the split energy of 0.14 eV. The triple and double B-H-B bridges are formed in B2H6+ and b-B3H7+, respectively. A new B-H-B bridge is formed while two B-B bonds are broken in B5H11+. The Jahn-Teller effect lowers the symmetry of B5H9 (C4v) to B5H9+ (C2) but slightly influences the structure of ara-B4H10 (C2v). The calculated properties of geometries, vibrational frequencies, and energies are compared with the experimental data available in the literatures.     

Ab initio molecular orbital calculations on the associated complexes of lithium cyanide with ammonia

Ab initio molecular orbital (MO ) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of NH₃ with Li⁺, C?N^?, LiCN, and its isomer LiNC. The BSSE -corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated (C_3v) structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia. © 1995 John Wiley & Sons, Inc.     

Ab initio molecular orbital calculations on large lattice cluster models: Use of translational symmetry

Summary Translational symmetry has been shown to be useful in the calculation of electronic structures of large lattice models. The number of unique integrals has been derived for cases of different dimensionality. For the unique integrals zero screening and approximation methods are described. The method has been applied to arrays of hydrogen atoms and to a zincblende surface model. When the size of the system is increased the translationally unique integrals are shown to become either zero or they can be calculated by simple coulombic approximations.     

Ab initio molecular orbital studies of polyethylene deformation

A molecular LCAO Hartree-Fock procedure was used to calculate total energies of axially stretched normal paraffins containing up to nine carbon atoms. The results are used to model the mechanical properties of polyethylene.     

Ab initio calculations on the molecular structure of fluorocyanopolyynes

The molecular structure of the first three members of the fluorocyanopolyynes was studied by ab initio Hartree-Fock calculations with a polarized double zeta basis set and at MP2 level with the same basis set. Alternating triple and single bonds were found; a theoretical estimate of rotational constants and dipole moments was performed and a comparison with the available experimental data was made. An analysis of the theoretical vibrational frequencies of the title compounds was carried out.     

Ab initio molecular orbital study of 1-fluorosilatrane

The molecular geometry of 1-fluorosilatrane was optimized fully by restricted Hartree–Fock (HF) calculations using the 3-21G, 3-21G(d) and 6-31G(d) basis sets, with the aim of locating the positions of the local minima on the energy hypersurface. The optimized geometries were compared with available experimental (X-ray and ED) and semiempirical data. The ab initio calculations using polarized basis sets are in good agreement with those of previously reported semiempirical calculations, giving a slightly longer equilibrium Si? N distance (～ 256 pm) in the case of the endo minimum. However, the exo minimum predicted by the semiempirical methods is not supported. There was no experimental evidence for the existence of this exo minimum, and the present ab initio calculations suggest that it is highly unstable. There is considerable disagreement among the experimental results in the C? N and C? C bond lengths in various silatranes, their differences being as large as 13 pm. The present calculations predict that these differences may appear because the silatrane skeleton is flexible with low-energy, large-amplitude internal motions which introduce considerable uncertainties into the position of ring carbon atoms. © 1994 by John Wiley & Sons, Inc.