Comparison of Theoretical Methods and Basis Sets for ab initio and DFT Calculations of the Structure and Frequencies of Normal Vibrations of Polyatomic Molecules

The dependence of the quality of calculation of the geometric parameters and frequencies of normal vibrations on the choice of the theoretical method and the basis set of Gaussian functions has been investigated within the framework of four approximations (DFT/B3LYP, HF, MP2, MP3), using benzene and s-triazine molecules as an example. It has been shown that the molecular parameters calculated using the basis set without polarization functions within the framework of any of the above theoretical methods agree poorly with the experimental data. It has been concluded that the use of the basis set 6-31G(d) within the framework of these methods with allowance for the electron correlation for calculating the geometric parameters and frequencies of normal vibrations of polyatomic cyclic compounds is most optimum in terms of the relation between the expenditure of time and the quality of the calculation. The coefficients of linear scaling of frequencies have been obtained by the DFT/B3LYP method for 22 basis sets that were tested on porphin, pyrrole, indene, and pyridine molecules. Atypically large errors in determining the frequencies of some benzene and s-triazine vibrations have been obtained in a number of quantum-mechanical calculations with large basis sets. The changes in the force field for these cases have been investigated with the example of the benzene molecule.     

Features of the electronic structure of the intermetallic Co3Ti in an atomically disordered state

Using a method based on incorporating statistics into the theory of multiple scattering via the distribution function of atoms on lattices, the electronic structure of the intermetallic Co₃Ti is calculated for both complete atomic order and complete disorder. An unusual feature of the electronic structure of this intermetallic when it is disordered in the solid state is identified. Possible consequences of this behavior are analyzed. Fiz. Tverd. Tela (St. Petersburg) 39, 809–810 (May 1996)     

Statistical mechanics of braided Markov chains: I. Analytic methods and numerical simulations

We investigate numerically and analytically the statistics of Markov chains on so-called braid (B _n ) and locally free (ℒℱ _n ) groups. Namely, we compute the mean length 〈μ〉 and the variance 〈μ²〉−〈μ〉² of the shortest word which remains after applying of all group relations to the randomly generatedN-letter word (Markov chain). We express the conjecture (numerically justified) that the mean value 〈μ〉 for the random walk on the groupB _n (n≫1) coincides with high accuracy with the same value for the random walk on the “locally free group weth errors” if the number of errors is of order of 20%.     

Preparation and Properties of Frost-Resistant Room-Temperature-Curable Compounds Based on Oligoethertetraurethane Diepoxides of Various Chemical Structures

New polytetraurethane diepoxides were prepared from oligo(tetramethylene oxide)diols of various molecular masses, various diisocyanates, and glycidol. High-strength frost-resistant cold-curable elastic compounds were prepared on their basis. The use of aminoethylpiperazine as a curing agent allows the cured materials to be prepared in 24 h at room temperature. The elastomers based on isophorone diisocyanate exhibit higher mechanical and thermal characteristics than those based on 2,4-toluene diisocyanate, which is due to higher degree of microphase segregation of soft and hard blocks of elastomers based in isophorone diisocyanate.     

Calculation of the vibronic spectra of adenine

The vibrational structure of the absorption spectra of the first two π-π* singlet transitions of adenine is calculated in the Franck-Condon approximation including Herzberg-Teller interactions. The effect of excitation-induced changes in molecular angles on the intensities of the vibrational components is estimated. Structural models of the adenine molecule in the excited states are constructed. The theoretical and absorption spectra of the first π-π* transition are compared. The results of the electronic structure calculations of adenine by different CNDO/S methods are discussed. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 334–344, March–April, 1997.     

Structural Characteristics and Ionic Composition of a Colloidal Solution of Silver Nanoparticles Obtained by Electrical-Spark Discharge in Water

Colloid Journal - This work is devoted to studying the structural and morphological properties of a silver colloidal solution obtained by the method of electrical-spark discharge in distilled...     

Semirings of cyclic types

The authors investigate semirings of cyclic types from the algebraic point of view. To simplify and facilitate the analysis, the local Fourier transform of these semirings is introduced. The authors describe zero divisors, nilpotent elements, invertible elements, idempotents, and the Jacobson radical. __________ Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 12, No. 2, pp. 175–192, 2006.     

On Anomalous Influence of an External Electric Field on a Second-Order Transition in a Thin Ferroelectric Film

Physics of the Solid State - The specific features of a phase transition in a thin ferroelectric film in an external electric film have been studied in terms of the...