The inclusion of correlation in the calculation of phosphorus NMR chemical shieldings

The effects of including correlation in the calculation of phosphorus nuclear magnetic resonance (NMR) chemical shielding has been investigated for a variety of molecules in the Hartree-Fock, second-order Møller-Plesset (MP2), and estimated infinite-order Møller-Plesset theory ab initio approaches in the gauge including atomic orbital (GIAO) scheme. The inclusion of correlation in the shielding calculations often leads to significant changes from the Hartree-Fock theory and provides results that are in improved agreement with experiment. © 1996 John Wiley & Sons, Inc.     

On the calculation of the exciton ZFS hamiltonian for organic molecular crystals

The method of calculating exciton ZFS constants and ZFS tensor principla axes in the theoretical framework of Sternlicht and McConnell is presented. The molecular ZFS hamiltonians are transformed into a common coordinate system and averaged, and the ZFS tensor is then diagonalized. Results are presented for benzene, naphthalene, anthracene, tetracene, phenanthrene, fluorene, and coronene, and compared with experimental values where available.     

Molecular ozone-water complex.ab initio calculation with inclusion of electron correlation

The geometrical and electronic structure of different molecular O₃–H₂O complexes has been calculated by the ab initio method in the 4–31G (d, p) basis set with inclusion of electron correlation according to the Möller-Plesset fourth-order perturbation theory (MP4). It has been shown that the geometrical structure of the experimentally observed hydrogen-bonded complex is mainly determined by the entropy (rather than energy) effect, and is characterized by an almost free internal rotation of the H₂O molecule.Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Ruberzhansk Department of Dnepropetrovsk Chemical Technological Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 3, pp. 26–30, May–June 1993.Translated by L. Smolina     

Molecular translational-motion correlation time in liquid benzene

A model-free method is used for determining dipole-dipole nuclear-spin interaction correlation times for liquids from the spin-lattice relaxation times on the basis of the molecular shape, the spin positions in the molecules, and the spin distribution in the liquid. An estimate has been made of the proton relative-motion correlation time for liquid benzene.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 504–506, July–August 1987.     

Hybridization and correlation in a model calculation of a two-electron bond

The paper presents a quantitative examination of some aspects of the molecular two-electron problem, using a calculation for a two-electron homonuclear bond based on a restricted set of one 2s and one 2p orbital per nucleus. The single-determinant approximations with pure 2s STO's and with hybrid AO's are considered, as well as “partial” configuration mixing (CI) over MO's involving one hybrid per atom and “complete” CI over the whole four-orbital basis. The calculations simulate an exact calculation as regards hybridization and (left-right) correlation effects. These are studied, for the lowest state, at various distances, introducing the axial electron density as a means for interpreting quantitatively the various effects. The importance of orthogonalizing the 2s AO's to the corresponding 1s AO's and the MO's used to the MO formed by 1s AO's is reviewed, pending further numerical analysis.     

水溶液中几种苯衍生物与β-环糊精包合物的热力学研究

β 环糊精 (β Ｃｙｃｌｏｄｅｘｔｒｉｎ ,β ＣＤ)是由 7个葡萄糖残基以α 1 ,4 糖苷键连接而成的环状化合物 ,具有疏水的空腔和亲水的外表面。这种特殊的结构使它可以作为宿主 (主体 )而依疏水作用力、范德华力或氢键包合多种客体分子形成包合物[1～ 3] ,并可增强客体分子对光、热、氧化等的稳定性[4,5] 。有机物质与 β ＣＤ形成包合物后通常会发生光谱等特征的变化 ,因此可用光谱等方法研究包合物的稳定常数[4] ,并进而获得水溶液中包合体系的热力学参数[6 ] 。本文采用紫外分光光度法 ,研究水溶液中几种苯衍生物与β ＣＤ包合物的稳定…     

Quantum-chemical calculation of interactions of two benzene molecules

Theoretical and Experimental Chemistry -     

Polarity of para-disubstituted benzene derivatives: a correlation analysis of the substituent effect

Russian Chemical Bulletin - The article is concerned with the influence of resonance donor substituents RD on the dipole moments μ of molecules 1,4-RDC6H4RA containing resonance acceptor...     

The calculation of frequency-dependent hyperpolarizabilities including electron correlation effects

The theory for the calculation of the frequency-dependent hyperpolarizabilities β(?2ω; 0, ω), β(?ω; 0, ω), and β(0; ω, ?ω) is discussed. New relations between these tensors are derived for those wave functions that obey the time-dependent Hellmann–Feynman theorem (e.g., the self-consistent field [SCF] or the exact wave function). Using second-order Møller–Plesset perturbation theory (MP2), expressions are obtained for the hyperpolarizabilities in terms of derivatives of appropriately defined linear polarizability tensors with respect to a static electric field. Results are presented for ammonia and formaldehyde for the optical Kerr effect and for secondharmonic generation. These results indicate that it is desirable to determine the frequency-dependent contribution to the hyperopolarizability at the MP2 rather than the SCF level of theory, in cases where the static hyperpolarizability has a large contribution from electron correlation and/or where the frequency-dependent contribution may be more significant, such as for secondharmonic generation.     

Theoretical study of the inclusion properties of 1,2,4,5-tetra(morpholinocarbonyl)benzene

Molecular inclusion by the new amide host molecule (TMB) has been reconsidered by calculating the crystal stabilization energies for the guest molecules in the TMB + guest system from the simple intermolecular potential functions of Caillet and Claverie. Water, ethylene glycol, methanol, and ethanol have been employed as guest molecules and their relative stabilities have been considered. Water has been found to be the most suitable guest molecule in the TMB + guest system. It also has been found that the guest host interaction is the most important contributor in determining the relative stabilities of the guest molecules in the TMB + guest system, but the guest guest interaction is very important, too. Moreover, the electrostatic interaction has been found to be the most important contributor to the total interaction energy in the TMB + guest system.     

Ab initio CI calculation of the molecular quadrupole moment of benzene

The molecular quadrupole moment of benzene has been calculated from ab initio CI wavefunctions of different levels of accuracy The calculated value from the wavefunction near the Hartree-Fock limit is reduced ≈10% in magnitude by the CI treatment.     

Time correlation function and finite field approaches to the calculation of the fifth order Raman response in liquid xenon

The fifth order, two-dimensional Raman response in liquid xenon is calculated via a time correlation function (TCF) theory and the numerically exact finite field method. Both employ classical molecular dynamics simulations. The results are shown to be in excellent agreement, suggesting the efficacy of the TCF approach, in which the response function is written approximately in terms of a single classical multitime TCF.     

Geometry optimizations with explicit inclusion of electron correlation

Automatic techniques for geometry optimization are applied in conjunction with configuration interaction and perturbation treatments of electron correlation. The computational effort and numerical accuracy of the optimizations are discussed, as well as problems with approximate correlation methods concerning the continuity of the potential surface. The optimized geometries of fourteen molecules obtained with different correlation treatments (MNDO SCF MOs) are compared. The configuration interaction results are reproduced satisfactorily by simple perturbation approaches. The largest change of the optimized SCF geometry is found for hydrogen peroxide.     

SCF CI calculation of the electronic spectra of nitroso and nitro benzene

SCF calculation with CI has been found to give a reasonable value for the longest wavelength - ^* transition in nitroso and nitro benzene if a sufficient number of configurations are taken into considerations. The applicability of this method to the case of n- ^* transition is, however, of doubtful validity. For the electron density approximation of chemical reactivity simple Hückel calculation may be good enough if the parameters are carefully chosen.

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Zusammenfassung Eine SCF-CI-Berechnung ergab brauchbare Werte für die langwelligsten - ^*-Übergänge von Nitro- und Nitrosobenzol, wenn genügend Konfigurationen in die Rechnung einbezogen werden. Jedoch ist der Wert der Methode für die Berechnung von n- ^*-Übergängen zweifelhaft. Die Elektronendichte ist nicht wesentlich von der der Hückel-Theorie verschieden, so daß letztere für Fragen der chemischen Reaktivität ausreichend erscheint, wenn die Parameter sorgfältig gewählt werden.

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Résumé Un calcul SCF donne des énergies raisonnables pour les premières transitions - ^* de nitroso- et de nitrobenzène, quand on considère un nombre suffisant de configurations. Mais l'application de ce procédé aux transitions n- ^* est douteuse. Si l'on croisit soigneusement les paramètres, la méthode simple de Hückel semble suffir pour le calcul des densités électroniques dans des problèmes de réactivité chimique.

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Sincere thanks are due to Prof. P. 0. Löwdin, Uppsala, Sweden, for laboratory facilities, where a part of the work was done and to Mr Klaus Appel for Machine Computation on Alwac-III at Uppsala.     

Structure reactivity correlation in inclusion complexes: Deoxycholic acid-thiocamphenilone

The 21 inclusion complex formed between deoxycholic acid (C₂₄H₄₀O₄,M _r =392.58) and thiocamphenilone (C₉H₁₄S,M _r =154.28) crystallizes in the space groupP2₁2₁2 witha=13.738(2),b=27.203(4),c=7.189(1) Å andZ=4. The structure was refined toR=0.158 andR _w =0.195 for 1649 observed reflections with |F _o |3.0|F _o |. The crystal structure is characterized by an assembly of anti-parallel pleated bilayers, formed by molecules of deoxycholic acid held together through hydrogen bonds. The guest thiocamphenilone occupies the crystallographic two-fold axis and is disordered. The orientation of the guest molecule obtained from crystallographic data is consistent with the results obtained from the potential energy calculations. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82042 (23 pages).     

The CNDO approximation in the Green's function method for VIP calculation

By introducing a CNDO adapted approximation into an appropriate irreducible interaction part a simple formula for calculation of vertical ionization potentials (VIP's) is derived. The method is applied to the molecules F₂, HF, C₂F₄, CH₂F₂, BF₃, CH₃F, CF₄.     

The 1:1 inclusion compounds zolmitriptan–benzene and zolmitriptan–phenol

In the benzene and phenol solvates of (S)‐4‐{3‐[2‐(dimethylamino)ethyl]‐1H‐indol‐5‐ylmethyl}oxazolidin‐2‐one, viz. C₁₆H₂₁N₃O₂·C₆H₆, (I), and C₁₆H₂₁N₃O₂·C₆H₅OH, (II), the host molecule has three linked residues, namely a planar indole ring system, an ethylamine side chain and an oxazolidinone system. It has comparable features to that of sumatriptan, although the side‐chain orientations of (I) and (II) differ from those of sumatriptan. Both (I) and (II) have host–guest‐type structures. The host molecule in (I) and (II) has an L‐shaped form, with the oxazolidinone ring occupying the base and the remainder of the molecule forming the upright section. In (I), each benzene guest molecule is surrounded by four host molecules, and these molecules are linked by a combination of N—H...N, N—H...O and C—H...O hydrogen bonds into chains of edge‐fused R₄⁴(33) rings. In (II), two independent molecules are present in the asymmetric unit, with similar conformations. The heterocyclic components are connected through N—H...N, N—H...O and C—H...O interactions to form chains of edge‐fused R₆⁴(38) rings, from which the phenol guest molecules are pendent, linked by O—H...O hydrogen bonds. The structures are further stabilized by extensive C—H...π interactions.     

On the calculation of correlation energies in the spin-density functional formalism

It is shown that dynamical correlation effects can be adequately treated using the local spin-density approximation. The computational effort is very small compared to CI calculations. The method is applied to correlation energies and ionization potentials of the atoms Li to Ar and binding energies of the diatomic hydrides LiH to HCl.     

Reverse nonequilibrium molecular-dynamics calculation of the Soret coefficient in liquid benzene/cyclohexane mixtures

Reverse nonequilibrium molecular dynamics is the method applied here for the investigation of thermal diffusion in realistic molecular fluids. The Soret coefficients of benzene/cyclohexane mixtures are calculated using an all-atom model. The autocorrelation functions indicate that the mole fraction gradient converges much slower than the temperature gradient. Compared to experimental data, the results show the same tendency of the Soret coefficient variation versus the mole fraction. Although a systematic error exists for the magnitude of the Soret coefficient, a meanwhile systematic error for both the mutual diffusion and thermal diffusion coefficients provides some explanation of it; and the calculation with different force field parameters indicates a possibility to annihilate the systematic error. The influences of algorithm variables such as cutoff lengths and perturbation intensities are tested. Furthermore the temperature dependence of the Soret effect is observed, yielding the same trend as previous studies.