Ab initio SCF computation of force constants for CO2

Force constants for CO₂ have been evaluated using SCF wave functions. The effect ofd basis functions and geometry are investigated. Comparison with experimental values shows that a large error, due to neglect of electron correlation, occurs for theK ₁₂ interaction stretch force constant.     

Ab initio computation of force constants: III. A simple procedure for the evaluation of X-H bond dissociation energies

A simple expression derived from the Morse potential allows X-H bond dissociation energies to be estimated from the quadratic stretching force constants obtained in ab initio computations. Agreement with experiment is excellent for C-H bonds, except when the radical is stabilized by π-conjugative effects, in which case the bond dissociation energy is overestimated. The procedure also fails to estimate ionic dissociation energies.     

Ab initio MO calculation of force constants and dipole derivatives for the formamide dimer. An estimation of hydrogen-bond force constants

Ab initio SCF MO calculations (with the 4-31G basis set) have been carried out to determine the equilibrium geometry, vibrational frequencies, dipole-moment derivatives, and force constants for intermolecular modes of the formamide dimer and its d₄ and d₆ derivatives. The results are correlated with monomer calculations and experimental data for crystalline formamide.     

Ab initio study of force constants and intensities of 1,3,5-trioxane

The structure, vibrational force constants and infrared and Raman intensities have been calculated for 1,3,5-trioxane using a 3-21G basis set. These results have been used to identify some possible inaccuracies in experimental diffraction based structures and in vibrational assignments. It is demonstrated that there is a marked contrast between the trends in the vibrational force constants and in the King's effective atomic charges of the axial and equatorial CH bonds in the series cyclohexane, 1,4-dioxane and 1,3,5-trioxane. The axial CH stretching force constant decreases by 0.04 mdyne Å⁻¹ for each adjacent oxygen atom, whereas that of the equatorial CH bond increases by 0.15 mdyne Å⁻¹ per oxygen. In trioxane the effective atomic charge of the axial hydrogen is twice that of the equatorial. Atomic polar tensors are calculated in a bond oriented frame, and the effect of the oxygens on CH stretching and bending mode intensities discussed. Some properties are also calculated using the 4-31G basis.     

Conformational properties of methyl vinyl sulfone: Ab initio geometry optimization and vibrational analysis

An ab initio conformational analysis of methyl vinyl sulfone (CH₂CHSO₂CH₃) has been carried out. Molecular geometry optimizations have been performed at the HF and MP2 levels of the theory. Relative energies of the stationary points have been determined by using different approaches, including electron correlation corrections up to the third order. The IR and Raman spectra of the liquid have been measured and a vibrational assignment is proposed. The height of the barrier to the methyl group internal rotation has been estimated. Theoretical calculations and vibrational spectra have shown that the predominant conformation of methyl vinyl sulfone has the C=C bond eclipsed with one of the S=O bonds. Similar eclipsed forms have been found in vinyl fluoro sulfone, vinyl chloro sulfone and divinyl sulfone by ab initio HF calculations.     

Ab initio study of spectroscopic constants and anharmonic force field of 74GeCl2

The equilibrium structure, spectroscopy constants, and anharmonic force field of germanium dichloride have been calculated at MP2, B3LYP, and CCSD(T) levels of theory employing two basis sets, cc-pVDZ and cc-pVTZ, respectively. The computed geometries, rotational constants, and vibration-rotation interaction constants, and quartic centrifugal distortion constants are compared with the available experimental data. The harmonic frequencies, anharmonic constants, and cubic and quartic force constants are predicted. The calculated results show that the MP2 results are in excellent agreement with experiment and represent a substantial improvement over the results obtained from B3LYP. The CCSD(T) method is also an advisable choice to study anharmonic force field of molecules.     

Ab initio calculations of the equilibrium geometry of cis and trans methylglyoxal using the force method

Ab initio calculations of the equilibrium ground-state conformation of trans-methylglyoxal, CH₃COCHO, have been carried out using the gradient method and a 4-3 IG basis set. The results are compared with experiment and with a theoretical geometry computed using an STO-3G basis set, and the more usual stepwise optimization of the parameters. The molecular orbital energies, dipole moment and ionization energy are calulated. Calculations are also carried out on the cis-isomer and the barrier to cis-trans isomerization is obtained.     

Ab initio calculation of resonance Raman cross sections based on excited state geometry optimization

A method for the calculation of resonance Raman cross sections is presented on the basis of calculation of structural differences between optimized ground and excited state geometries using density functional theory. A vibrational frequency calculation of the molecule is employed to obtain normal coordinate displacements for the modes of vibration. The excited state displacement relative to the ground state can be calculated in the normal coordinate basis by means of a linear transformation from a Cartesian basis to a normal coordinate one. The displacements in normal coordinates are then scaled by root-mean-square displacement of zero point motion to calculate dimensionless displacements for use in the two-time-correlator formalism for the calculation of resonance Raman spectra at an arbitrary temperature. The method is valid for Franck-Condon active modes within the harmonic approximation. The method was validated by calculation of resonance Raman cross sections and absorption spectra for chlorine dioxide, nitrate ion, trans-stilbene, 1,3,5-cycloheptatriene, and the aromatic amino acids. This method permits significant gains in the efficiency of calculating resonance Raman cross sections from first principles and, consequently, permits extension to large systems (>50 atoms).     

Ab initio rotational constants of interstellar species: Cyanoacetylene hydrogenated derivatives

By means of ab initio calculations, the rotational constants and dipole moments of H_nC₃N (n = 1, 3, 5, 7, and 9) species have been calculated at the HF / 6-31G * level of theory. Selected cases have been also calculated at the MP 2/6-31G * level and the influence of calculation level on rotational constant values is briefly discussed. Some of these species were discovered in the interstellar medium, while others have still not been detected there, although their existence is very probable. The results given here could help in their detection. © 1993 John Wiley & Sons, Inc.     

Ab initio study of the NMR shielding constants and spin-spin coupling constants in cyclopropene

Summary Ab initio calculations of parameters which characterize the NMR spectrum are presented for the cyclopropene molecule. The London orbitals CHF (or GIAO-CHF, Gauge-Independent Atomic Orbital Coupled Hartree-Fock) results for the shielding constants are in good agreement with the experimental data, accurately determined, and with otherab initio values. The calculations of the NMR spin-spin coupling constants have been performed using the Multiconfiguration Time-Dependent Hartree-Fock (MC TDHF) approach. Different basis sets and MC SCF wavefunctions were used to estimate the accuracy of the results. Good agreement is obtained with the coupling constants estimated using the available experimental data.Dedicated to Professor Werner Kutzelnigg on the occasion of his 60th birthday     

Conformational analysis of the methyl ester of alanine by gas electron diffraction and Ab initio geometry optimization

The gas electron diffraction data of the methyl ester of -alanine were recorded at an approximate nozzle temperature of 80°C and analyzed with geometrical constraints taken from 4-21G ab initio gradient geometry refinements. The molecular intensities are consistent with a model for the most populated conformational state in which the N---C---C=O torsion (τ) is syn-peri-planar and a bifurcated hydrogen bond exists between NH₂ and C=O. Details of the intensity data make it impossible to rule out the presence of significant concentrations of a second conformer, (τ + 150°), in contrast to the ab initio calculations which predict greater stability of the system at τ −150° than +150°.     

Ab initio computation of vibrational relaxation rate coefficients for the collisions of CO2 with helium and neon atoms

Ab initio calculations of rate coefficients are reported for the vibrational relaxation of CO₂ molecules in collision with helium and neon atoms. Self consistent-field computations have been performed to parameterise simple three-dimensional potential energy functions which have been used in vibrational close-coupling, rotational infinite-order-sudden calculations of rate coefficients. Excellent agreement is obtained between the calculated and experimental rate coefficients for the deactivation of the (01₁0) vibrational level in the He + CO₂ system at temperatures of 300 K and above. The ab initio predictions of rate coefficients for relaxation of CO₂ vibrational levels such as (10⁰0) and (02⁰0) should be useful in computer simulations of CO₂ lasers.     

Ab initio force field for simulations of proteins and nucleic acids

We present a new self-consistent set of ab initio analytical pair potential to predict specific nonbonded interactions of protein with nucleic acid, of protein with protein, and of nucleic acid with nucleic acid. The purpose of this study is to represent the interaction between biological molecules with an accuracy equivalent to the ab initio molecular orbital calculations, which are used as reference data to obtain the pair potentials. Atoms in nucleic acids and proteins are classified according to their chemical environments. An “effective charge,” a modification of a charge obtained from the Mulliken population analysis, is introduced and used to represent the electrostatic energy. More than 30,000 SCF interaction energies have been calculated to provide the reference data for the fitting procedure that we have adopted in the parameterization of the potentials. The standard deviation is 1.61 kcal/mol for interaction energies spanning the range from about ?220 kcal/mol to +20 kcal/mol. Molecular dynamics simulations, using the above new set of force field, have been performed successfully for the systems where adequate treatments of specific interactions are required: The stability of α-helix of C-peptide and the interaction of spermine with oligonucleotide are examined as preliminary examples.     

Ab initio molecular orbital theory study of GaAs clusters: The geometry

We have studied the structure and geometry of neutral and charged atomic clusters consisting of Ga and As atoms via ab initio Hartree–Fock (HF) and second‐order Møller–Plesset methods. The Ga_mAs_n cluster with m≠n composition prefers a nontetrahedral geometry in the charge neutral (q=0) state. These clusters tend to be stable in tetrahedral geometry when appropriately charged. The Ga_mAs_n cluster with m=n composition (1:1 ratio of Ga to As atoms) tends to be stable in a tetrahedral geometry in the charge neutral (q=0) state. With increasing size of the cluster, the geometry of Ga_nAs_n cluster approaches the zinc‐belende‐type crystalline structure. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 563–573, 2000     

Ab initio study of the molecular geometry and properties of nitrosyl fluoride

Ab initia SCF MO calculations employing a basis set of multi-component Gaussian orbitals are carried out for the FNO molecule at various internuclear angles and FN bond separations. The resulting total energy at the experimental FNO geometry is estimated to be in the range of 0.15 to 0.25 hartrees above the Hartree-Fock limit; the calculations find an internuclear equilibrium angle which is 1.1 higher than the experimental value (110) and a FN bond distance which is 0.02 bohrs greater than that found experimentally (2.872 bohrs). The molecular charge density contours indicate that nitrosyl fluoride can be considered quite concisely as a loosely bound combination of a fluorine atom and an NO radical. Finally, a model is constructed on the basis of the SCF calculations which is capable of explaining rather anamolous features of the FNO molecular geometry and properties in a consistent manner.

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Zusammenfassung Für das FNO-Molekül wurden bei verschiedenen Molekülwinkeln und FN-AbstÄnden ab initio SCF-MO-Berechnungen durchgeführt, wobei als Basis Atomfunktionen, welche sich jeweils aus einer grö\eren Anzahl von Gau\funktionen zusammensetzen, verwendet wurden. Dabei ergibt sich für das Molekül in der experimentellen Gleichgewichtsstruktur eine Gesamtenergie, welche als ungefÄhr 0,15 bis 0,25 Hartree über der Hartree-Fock Energie liegend abgeschÄtzt wird; die Berechnungen ergeben einen Wert für den Molekülwinkel, der 1,1 grö\er ist als der experimentelle (110) und eine um 0,02 Bohr grö\ere FN-BindungslÄnge als experimentell ermittelt worden ist (2,872 Bohr). Die berechnete Elektronendichteverteilung im Molekül deutet darauf hin, da\ FNO als Gebilde betrachtet werden kann, das aus einem Fluoratom und einem NO-Badikal besteht, welche nur locker aneinander gebunden sind. Auf Grund der SCF-Rechnungen wird weiterhin ein Modell entworfen, welches in der Lage ist, die Besonderheiten in der Struktur und den Eigenschaften des FNO-Moleküls auf konsequente Weise zu erklÄren.

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Résumé La molécule FNO est calculée par la méthode ab-initio SCF MO, utilisant une base d'orbitales gaussiennes multiples, pour différents angles et différentes distances FN. L'énergie totale obtenue pour la géométrie expérimentale est considérée comme se trouvant à 0,15-0,25 hartrees au dessus de la limite Hartree-Fock; les calculs fournissent un angle d'équilibre supérieur de 1,1 à la valeur expérimentale (110) et une distance F-N supérieure de 0,02 bohrs à la distance expérimentale (2,872 bohrs). Les contours de densité de charge montrent que le fluorure de nitrosyle peut Être considéré avec précision comme une combinaison entre un radical NO et un atome F faiblement lié. Sur la base des calculs SCF on construit un modèle capable d'expliquer raisonnablement des caractères anormaux de la géométrie et des propriétés de FNO.

     

Effects of hydration on scale factors for ab initio force constants

Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To this end, we have calculated scale factors for ab initio force constants for formic acid, acetic acid, and acetone using a least squares fit of calculated and experimental frequencies. We compare these scale factors with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G basis. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. The results indicate that scale factors can simulate the effects of solvation on molecular force constants to yield accurate scaled ab initio force fields.     

On the ab initio geometry optimization of molecular solutes

We present several variants of methods for the automatic search of optimum geometries of solutes via ab initio SCF procedures. The physical meaning of geometry optimization in solution is discussed. Advantages and disadvantages of the different variants are shown making use of calculations on the HF dimer with different basis sets, supplemented by information on the computational times. Suggestions for the most convenient strategies (which in part depend on the nature of the solute) are also done.