Implementation of gradient-optimization algorithms and force constant computations in BSSE-free direct and conventional SCF approaches

A modification of the Roothaan equations was described in a previous work, which aimed to avoid the BSSE at the Hartree–Fock level of theory. The resulting scheme was called the self-consistent field for molecular interactions (SCF-MI) to underline its special usefulness in the computation of intermolecular interactions. The method provides a complete a priori elimination of the BSSE, while taking into account the natural nonorthogonality of the MOs of the two interacting fragments. Compatibility with the usual formulation of the analytic derivatives of the SCF energy is also guaranteed. This allowed the implementation of gradient-optimization algorithms and force constant matrix computations in both the direct and conventional SCF approaches. The SCF-MI method has been incorporated into the GAMESS-US package. Tests have been performed at the Department of Chemistry of the Iowa State University. Increases in the complication and computation time are minimal if compared to standard SCF codes and the method shows much less basis-set dependence in the predicted molecular properties. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 151–158, 1998     

Derivatives of molecular Vibrational force constants with a singular Jacobian

Counter examples are presented that negate the generality of Gans' claim that a singular jacobian demands the existence of one off-diagonal force constant F_st possessing the extremal property ?F_rr/?F_st = ∞ for all of the n diagonal force constants F_rr (r = 1, > 62; n) in a symmetry species containing n vibrations.     

Viscoelasticity of a stretched polymer chain with ends exposed to a constant force

The viscoelasticity of a stretched polymer chain with its end particles exposed to oppositely acting forces is studied via collisional molecular dynamics and analytically. A simple model according to which a polymer molecule is a chain of particles linked through freely jointed elastic bonds is adopted. The analytical theory is in good agreement with the results of the computer simulation of time correlation functions in the range of large-scale motions of a polymer molecule. It is found that the decay of correlation functions K _αβμν of fluctuations of the microscopic stress tensor of a chain, K _zzzz, K _zαzα , = K _zαzα , (α = x, y; z is the axis along which the forces act), is slowed down, and their value increases relative to the respective correlation functions of a chain with fixed ends. The greater the force, the higher this difference. The correlation functions that are transverse with respect to the z axis do not differ from those for chains with fixed ends. The results show that, in the calculation of time correlation functions of strongly stretched polymer chains, different statistical ensembles are not equivalent; this must be taken into account in the dynamic theories of heavily deformed polymers.     

The conformation and vibrational spectra of trans-1,4-dihalocyclohexanes: Part IV. Force constant calculations and spectral correlations

A force field is derived by the overlay technique, iterating on the 48 assigned fundamentals for the ee and the aa conformers of six trans-1,4-dihalocyclohexanes. Some additional fundamentals of three monohalocyclohexanes (chloro-, bromo- and iodocyclohexane) are used in the iterations. The final force field has 49 independent parameters; it is compatible with the Snyder and Schachtschneider valence force field on n-alkyl chlorides, and is adjusted to more than 600 observed frequencies. Practically all the force constants are common to the ee and aa conformers and the force field can be easily transferred to other substituted cyclohexanes.The assigned fundamentals for the six trans-1,4-dihalocyclohexanes (dichloro-, dibromo-, diiodo-, chlorobromo-, chloroiodo- and bromoiodocyclohexane) are correlated and the spectral features discussed. A few new assignments are made for trans-1,4-dichlorocyclohexane.     

Passivity of titanium,part IV: reversible oxygen vacancy generation/annihilation

Journal of Solid State Electrochemistry - A simplified Point Defect Model incorporating reversible oxygen vacancy generation/annihilation at the metal/film interface has been used to investigate...     

Bending force constant formulas from a bond-charge model of triatomic molecules

A bond-charge point-dipole model of molecular electronic structure is used to derive a general relation between the bending force constant, K_θθ, and stretching force constant , K₁₁, of linear and non-linear triatomic molecules.     

Force constant calculations for octahedral complexes of the type M(CO)5L, based on the CO-factored force field

Using the maximum value of the sum of the interaction constants, the CO-factored force field of octahedral M(CO)₅L molecules was solved to give the relations which allow direct calculation of force constants from C---O stretching frequencies of the all-¹²C¹⁶O molecule. The force constants calculated by these relations were used to predict C---O stretching frequencies of isotopically enriched species of some complexes of the type M(CO)₅L. The results obtained showed that there exists a very good agreement between observed and calculated frequencies. This led us to conclude that the solution presented for the force field of M(CO)₅L molecules is a valid solution.     

Calcium-lead-copper and calcium-lead-cadmium hydroxylapatite solid solutions: Preparation,infrared and lattice constant measurements

Summary Homogeneous solid solutions of Ca/Pb/Cu hydroxylapatites, keeping the Ca content fixed, and Ca/Pb/Cd hydroxylapatites, maintaining the Cd amount fixed, have been prepared over the entire Cu/Pb and Ca/Pb composition ranges respectively by the method of co-precipitation in aqueous media. I.r. frequencies and lattice constants of the solid solutions have been obtained and are found to vary linearly with composition.     

Relation of the force constant of a bond to the electric field at a nucleus

The change in the electric field at a nucleus in a molecule due to bond stretch is related to the force constant of the stretched bond. The validity of this relationship using approximate wave functions at the SCF and MP2 levels of theory is tested for the diatomic molecules H₂, HF, CO, and N₂. The effect of basis set variation on H₂ is also investigated. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1664–1667, 1997     

Behavior of polyelectrolyte solutions in a wide range of solvent dielectric constant

We exploit the strong temperature dependence of dielectric constant of N-methylformamide to control the strength of electrostatic interactions in polyelectrolyte solutions of sodium polystyrene sulfonate, and investigate the dynamic properties of salt-free solutions over a broad temperature range, from 54 to −58 °C. The traditional fast and slow diffusion processes are observed. The ratio of diffusion coefficients, D_s/D_f, increases and the ratio of their amplitudes, A_s/A_f, decreases, both by a factor of about two in this temperature range confirming the expected temperature variation with the Bjerrum length.     

Correlations of substituent parameters with the carbonyl stretching force constant in arenetricarbonylchromium complexes

Correlations are established between selected substituent parameters (σ_I,σ_P, σ_R^P,σ_P^O,σ_R^O) and the carbonyl stretching force constant, k(CO), for 28 mono and poly-substituted tricarbonylchromium-complexed arene compounds. On the basis of the statistical results it is concluded that the overall electronic substituent effect transmitted to the carbonyl groups involves both mesomeric and inductive mechanisms. Within the restricted domain, including substituent group and benzene ring, transmission proceeds largely by resonance, with a minor inductive (through-bond and field) effect operative in the same domain. Further transmission from the substituted arene ring to the chromium atom predominantly involves an inductive mechanism. This result, in support of existing literature data, suggests appreciable participation of the ring carbon σ framework in the metal—ring bond formation.     

Statistical theory of solutions of salts in solvents with low dielectric constant

The statistical theory of solutions according to McMillan and Mayer is applied to solutions of salts in solvents with low dielectric constant. It is shown that the irreducible integrals B₂₀ and B₁₁, which characterize the interaction between two molecules of solvent and between a molecule of solvent and a molecule of dissolved substance, respectively, can be calculated through the partial molar volumes of the components and the isothermal compressibility of the solvent. Their contribution to the second virial coefficient is extremely small for solutions of salts in media with low dielectric constant. By means of a continuum model, the B₀₂ values are calculated as functions of the parameter x=²/DkT³, and an explanation is given for the relationships observed in the association of the salts. An approximate calculation of the effective diameter of the molecules x=²/DkT³ is given for some salts.     

The relations of bond length and force constant with the potential acting on an electron in a molecule

Within a molecule, the potential acting on an electron (PAEM) is defined as the interaction energy of a local electron with the rest of the particles, i.e., all nuclei and remaining electrons. The formalism of the PAEM is first derived, and the calculated PAEMs are then obtained by using the ab initio program based on the MELD program package for a series of diatomic halides, namely, HX, LiX, NaX, and X(2) molecules (X= F, Cl, Br, and I), as well as H(2)O and NH(3), and some organic molecules. By comparing the 3D topological graphs of the PAEMs, we found that there is a saddle point along every chemical bond axis. Further, the good linear correlations of the force constant and bond length with the PAEMs are explored through a definition of D(pb), which is the absolute value (i.e., the negative) of the PAEM at the saddle point along the chemical bond axis. In addition, the difference between the PAEM and molecular electrostatic potential (MEP) is pointed out by analyzing both their definitions and numerical results.     

Capillary constant and surface tension of methane-nitrogen solutions: 1. Experiment

The differential version of the method of capillary rise has been used to measure the capillary constant and calculate the surface tension of methane-nitrogen solutions. Experiments have been conducted in the temperature range from 95 to 170 K at pressures up to 4 MPa. Experimental data on surface tension have been compared with the results of calculations by thermodynamic models. Equations are given which describe the dependence of the capillary constant of a solution on its temperature and composition.     

The chemistry of drugs part IV,Configurations of antimalarials derived from qinghaosu: Dihydroqingyhaosu,artemether, and artesunic acid

The structures of the antimalarials dihydroqinghaosu (2) , artemether (3) , and artesunic acid (7a) derived from qinghaosu were elaborated by ¹ H-NMR spectroscopy, and supported with X-ray data obtained for 2 and 3 . Several new derivatives, useful for the chemical characterization of dihydroqinghaosu (2) and artesunic acid (7a) , were prepared.     

Microdrops on atomic force microscope cantilevers: evaporation of water and spring constant calibration

The evaporation of water drops with radii approximately 20 microm was investigated experimentally by depositing them onto atomic force microscope (AFM) cantilevers and measuring the deflection versus time. Because of the surface tension of the liquid, the Laplace pressure inside the drop, and the change of interfacial stress at the solid-liquid interface, the cantilever is deflected by typically a few hundred nanometers. The experimental results are in accordance with an analytic theory developed. The evaporation process could be monitored with high accuracy even at the last stage of evaporation because (1) cantilever deflections can be measured with nanometer resolution and (2) the time resolution, given by the inverse of the resonance frequency of the cantilever of approximately 0.3 ms, is much faster than the typical evaporation time of 1 s. Experimental results indicate that evaporation of the last thin layer of water is significantly slower than the rest of the drop, which can be due to surface forces. This drop-on-cantilever system can also be used to analyze the drop impact dynamics on a surface and to determine the spring constant of cantilevers.     

Tilt of atomic force microscope cantilevers: effect on spring constant and adhesion measurements

In atomic force microscopy, the cantilevers are mounted under a certain tilt angle alpha with respect to the sample surface. In this paper, we show that this increases the effective spring constant by typically 10-20%. The effective spring constant of a rectangular cantilever of length L can be obtained by dividing the measured spring constant by cos2 alpha(1 - 2D tan alpha/L). Here, alpha is the tilt angle and D is the size of the tip. In colloidal probe experiments, D has to be replaced by the radius of the attached particle. To determine the effect of tilt experimentally, the adhesion force between spherical borosilicate particles and planar silicon oxide surfaces was measured at tilt angles between 0 degrees and 35 degrees. The experiments revealed a significant decrease of the mean apparent adhesion force with a tilt of typically 20-30% at alpha = 20 degrees. In addition, they demonstrate that the adhesion depends drastically on the precise position of contact on the particle surface.     

Reactions of nitroxides with sulfur‐containing compounds,part IV: Synthesis of novel nitroxide (thio)ureas1

The reactions of 4‐isothiocyanato‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl 2 and 4‐isocyanato‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl 3 with selected amines and lower alcohols give the corresponding novel thioureas 4 , ureas 5 , thiocarbamates 6 , and carbamates 7 , all bearing the nitroxyl moiety. The characteristic features of EI mass spectra of (thio)ureas 4 and 5 are described. Some of the synthesized thioureas 4 , ureas 5 , thiocarbamates 6 , and carbamates 7 are moderately or weakly active against pathogenic fungi. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:393–401, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20228     

Crystallographic and structural characterization of heterometallic platinum compounds part IV: heterotetranuclear clusters

This review includes over two hundred heterotetranuclear platinum clusters. The clusters are of the compositions Pt₃M, Pt₃M₂, PtM₃, Pt′₂MM′, PtM₂M′ and PtMM′M”. There are twenty five different M atoms (transition and non-transition) as a partner(s) of platinum. The four metal atoms are found in a tetrahedral, planar-rhombohedral, butterfly, spited-triangular, cubane, eight — and oligo-membered rings and a unique structures. There is wide variety of the ligands from uni to- undecadentate, with the most common P and C donor sites. The shortest Pt-M (transition) versus Pt-M (non-transition) bond distances are 2.4833(8)Å (M=Pd) vs. 2.4365(5)Å (Ge). Several relationships between the various structural parameters were found and are discussed.