Showing papers on "Dipole published in 1987"

Atomic charges derived from electrostatic potentials: A detailed study

Abstract: A new algorithm for fitting atomic charges to molecular electrostatic potentials is presented. This method is non-iterative and rapid compared to previous work. Results from a variety of gaussian basis sets, including STO-3G, 3-21G and 6-31G*, are presented. Charges for a representative collection of molecules, comprising both first and second row atoms and anions are tabulated. The effects of using experimental and optimized geometries are explored. Charges derived from these fits are found to adequately reproduce SCF dipole moments. A small split valence representation, 3-21G, appears to yield consistently good results in a reasonable amount of time.

Tests of the helix dipole model for stabilization of α-helices

Abstract: Charged groups play a critical role in the stability of the helix formed by the isolated C-peptide (residues 1-13 of ribonuclease A) in aqueous solution. One charged-group effect may arise from interactions between charged residues at either end of the helix and the helix dipole. We report here that studies of C-peptide analogues support the helix dipole model, and provide further evidence for the importance of electrostatic interactions not included in the Zimm-Bragg model for alpha-helix formation.

Phenomenological theory of optical second- and third-harmonic generation from cubic centrosymmetric crystals

Abstract: We present a macroscopic theory for anisotropic second- and third-harmonic generation obtained in reflection from the surface and bulk of cubic centrosymmetric single crystals. The theory is based on possible electric dipole, electric quadrupole, and magnetic dipole sources. Completely general expressions for the harmonic fields are obtained for (100), (111), and (110) faces independent of the details of the surface response but consistent with crystal symmetry. The results obtained agree with all existing experimental data obtained by various groups during the past few years. The possibility of separating out surface and bulk responses is considered using symmetry, polarization, or geometry arguments and it is concluded that for second-harmonic generation this cannot be done in general without additional information. Third-harmonic generation, barring any strong resonantly enhanced surface electric dipole effects, is essentially a bulk probe.

Atomic data for opacity calculations. II. Computational methods

Abstract: For pt.I see ibid., vol.20, p.6363-78 (1987). A general description of the data requirements for opacity calculations has been given in paper I. The present paper gives a detailed description of the methods being used in a collaborative effort which is referred to as the Opacity Project. The close-coupling approximation of electron-atom collision theory is used to calculate energies and wavefunctions for bound states, oscillator strengths, photoionisation cross sections and parameters for line broadening by electron impact. The computations are made using the R-matrix method together with new codes for calculating outer-region solutions and dipole integrals. Use of these techniques provides an efficient means of calculating large amounts of accurate atomic data.

Improving the validity of hyperfine field distributions from magnetic alloys: Part I: Unpolarized source☆

Abstract: The evaluation of magnetic hyperfine field distributions from 57Fe Mossbauer spectra in systems with both magnetic dipole and electric quadrupole interactions is considered. This problem is treated in higher order perturbation theory, and the case of general magnetic texture and isotropic electric quadrupole texture is treated in full. Simple expressions are given enabling the calculation of all spectrum line positions, intensities and widths. It is shown that in cases where first order perturbation treatments yield low-field artifacts in the magnetic hyperfine distribution, this procedure yields much more reliable results. It is also shown that the calculated magnetic spin texture is much more reliable. The asymmetry effects from the electric quadrupole interaction and from a correlation between this and a dipole magnetic field are considered.

A molecular dynamics simulation of a water model with intramolecular degrees of freedom

Abstract: Simulations of water models with and without intramolecular degrees of freedom have been performed within the classical approximation. Three H2O simulations with interaction parameters taken from a simple point charge model due to Berendsen et al. are reported: (i) with rigid water molecules, (ii) with flexible water molecules and (iii) with rigid water molecules and polarized bond lengths and bond angles obtained from the second simulation. The most important effect of the inclusion of intramolecular vibrations is seen on transport properties, which in general become a foctor of 2-3 faster. The internal vibrations lead to a small but significant lowering of the potential energy and also reduce the structure, as seen from atom-atom distribution functions. The dipole moment fluctuations are affected by the nuclear polarization caused by the intermolecular interactions. Two D2O simulation have been performed in order to assess the effects of isotope substitution, one with rigid molecules and the other with ...

Analytic evaluation of energy gradients for the single and double excitation coupled cluster (CCSD) wave function: Theory and application

Abstract: The theory for the analytic evaluation of energy gradients for coupled cluster (CC) wave functions is presented. In particular, explicit expressions for the analytic energy gradient of the CC singles and doubles (CCSD) wave function for a closed‐shell restricted Hartree–Fock reference determinant are presented and shown to scale as N6 where N is the one‐electron number of atomic basis functions for the molecular system. Thus analytic CCSD gradients are found to be of the same magnitude in computational cost as is the evaluation of analytic gradients for the configuration interaction singles and doubles (CISD) wave function. Applications of this method are presented for the water molecule and the formaldehyde molecule using a double‐ζ plus polarization (DZ+P) basis set. The CCSD equilibrium geometries, dipole moments, and, via finite differences of gradients, CCSD harmonic vibrational frequencies and infrared intensities are reported. For H2O these results are compared to analogous CISD, CISDT, CISDTQ, and...

Electric Dipole Tracing in the Brain by Means of the Boundary Element Method and Its Accuracy

Abstract: A method of localizing an electrical dipole in the brain from the scalp potential distribution has been developed with the aid of the boundary element method, in which a real geometry of the head is exactly taken into account and homogeneous electrical conductivity is assumed. Accuracy of the method was evaluated through animal experiments with a cat in which a current dipole was artificially generated in the brain. Deviation of the estimated dipole location from the true one was not random, but rather systematic (probably due to in-homogeneous conductivity distribution). It is numerically found that cavities in the skull disturb the inverse solution especially when the dipole is oriented toward the cavities. In vivo tests of the method were also done for primary somatosensory evoked potentials as a response to median nerve stimulation of a cat and myoclonic EEG. Although the homogeneous approximation was made, it does not change the significance of the results obtained by the present method.

Hyperfine fields of 3d and 4d impurities in nickel.

Abstract: We present detailed calculations of the electronic structure and the hyperfine fields of 3d and 4d impurities in nickel. The calculations are based on the local-density approximation of density-functional theory and the Korringa-Kohn-Rostoker Green's-function method for impurity calculations. We self-consistently calculate the local moments and hyperfine fields of the impurities and their nearest neighbors. We derive new formulas for the proper relativistic generalizations of the contact, orbital, and dipolar contributions to the hyperfine field and explicitly calculate relativistic corrections to the contact interaction which are important for 4d impurities. The hyperfine fields can be split up into local and transferred contributions which are directly related to the local moments and to the moments of the neighboring atoms. The calculated hyperfine fields are in reasonable agreement with the experimental data.

Fluorescence emission at dielectric and metal-film interfaces

Abstract: It is well known that the classical optical properties of a bare or metal-film-coated dielectric surface significantly the emission pattern of a fluorophore in close proximity to it. Most previous classical calculations of this perturb model the fluorophore as a continuous fixed-amplitude dipole acting as a simple radiator. However, for effect modeling steady-state excitation, a fixed-power dipole is more appropriate. This modification corresponds to normalizing fixed-amplitude dipole intensities by the total dissipated power, which is itself dependent on fluorophore orientation and proximity to the surface. The results for the fixed-power model differ nontrivially from the fixed-amplitude model. Using the fixed-power dipole model, we calculate the observation-angle-dependent intensity as a function of the fluorophore’s orientation and distance from the surface. The surface can have an intermediate layer of arbitrary thickness on it, which is used to model a metal-film-coated dielectric. In addition, general expressions are derived for the emission power as observed through a circular-aperture collection system (such as a microscope objective) located on either side of the interface. These expressions are applied to several common cases of fluorophore spatial and orientational distributions at bare glass–water and metal-film-coated glass-water interfaces. The results suggest practical experimental approaches for measuring the spatial and orientational distribution of fluorophores adsorbed at a surface, utilizing the distance-dependent fluorescence near a metalized surface and optimizing the collection efficiency from a well-defined volume near a quenching surface.

Collective dipole oscillations in small sodium clusters

Abstract: Photoabsorption cross sections of small neutral sodium clusters composed of N=2--40 sodium atoms are measured by longitudinal--beam-depletion spectroscopy at several wavelengths of visible light. Absorption occurs via coupling of photons to collective oscillations of the valence electrons. The cross section is strongly size and wavelength dependent. Good agreement is found with predictions based on an extended Clemenger-Nilsson shell model and the experimental static polarizabilities.

Molecular dynamics of the water liquid-vapor interface

Abstract: The results of molecular dynamics calculations on the equilibrium interface between liquid water and its vapor at 325 K are presented. For the TIP4P model of water intermolecular pair potentials, the average surface dipole density points from the vapor to the liquid. The most common orientations of water molecules have the C2 nu molecular axis roughly parallel to the interface. The distributions are quite broad and therefore compatible with the intermolecular correlations characteristic of bulk liquid water. All near-neighbor pairs in the outermost interfacial layers are hydrogen bonded according to the common definition adopted here. The orientational preferences of water molecules near a free surface differ from those near rigidly planar walls which can be interpreted in terms of patterns found in hexagonal ice 1. The mean electric field in the interfacial region is parallel to the mean polarization which indicates that attention cannot be limited to dipolar charge distributions in macroscopic descriptions of the electrical properties of this interface. The value of the surface tension obtained is 132 +/- 46 dyn/cm, significantly different from the value for experimental water of 68 dyn/cm at 325 K.

Properties of atoms in molecules: Dipole moments and transferability of properties

Abstract: This paper uses the theory of atoms in molecules to investigate the origin of molecular dipole moments. The dipole moment is given by a sum over the net charge and first moment of every atom in a molecule. The first term leads to a charge transfer contribution μc, the second to an atomic polarization contribution μa. It is shown that both terms are, in general, of equal importance in determining both the static molecular dipole moment and the moment induced by a nuclear displacement. Models which imploy only point charges and corresponding bond moments which follow rigidly the nuclear framework, i.e., models which approximate μc and ignore μa, are shown to lead to results that are incompatible with the changes that are found to occur in a molecular charge distribution during a nuclear vibration. The dipole moment is shown to be another group property that is transferable between molecules in the normal hydrocarbons, This property, along with the net charge, the energy, the correlation energy (expressed as...

Method and system for measuring azimuthal anisotropy effects using acoustic multipole transducers

Abstract: An acoustic borehole well logging method and apparatus for measuring azimuthal anisotropy of a formation traversed by a borehole using at least one multipole transducer. In a preferred embodiment, a dipole wave transmitter and at least one detector sensitive to dipole waves are employed. In an alternative preferred embodiment, a monopole transmitter and at least one multipole detector are employed. In the inventive method, two acoustic wave arrivals are detected, each associated with a different azimuthal orientation relative to the longitudinal axis of the borehole (i.e. each is transmitted by a multipole transmitter oriented at such angle, or is detected by a multipole detector oriented at such angle, or both). The inventive apparatus preferably includes at least one transducer unit including two or more multipole transmitters (or two or more multipole detectors) oriented at different azimuthal angles relative to the tool's longitudinal axis.

Monte Carlo simulation of liquid–liquid benzene–water interface

Abstract: A Monte Carlo simulation of liquid–liquid benzene–water interface is reported. Molecular pair potentials obtained from quantum mechanical calculations have been used. The two phases remain stable over 70 000 configurations/molecule and the interior of each phase is shown to have bulk properties. The intrinsic interface is molecularly sharp, but the interfacial region is broadened by capillary waves. The preferential water–benzene orientation at the interface is similar to that in a dilute aqueous solution of benzene. There is no longitudinal water ordering induced by the interface. The presence of the nonpolar phase gives an alignment of water dipoles parallel to the surface and a reduced probability of parallel orientation of adjacent dipoles. The hydrogen bonds between water neighbors are reinforced at the interface.

Molecular dynamics simulation of dielectric properties of water

Abstract: The dielectric properties of liquid water are studied using an empirical potential model treating the monomers as flexible molecules with short‐range interactions between oxygens and Coulomb interactions between charges situated on the individual atoms. Satisfactory results for the static dielectric constants at 259, 300, and 350 K are obtained from long simulation runs; these constitute a significant improvement over recent calculations using other potential models. Time correlation functions of dipole moment and single molecule orientation are evaluated. In the supercooled state an enhanced slowing down in the temporal decay of polarization fluctuations occurs, a behavior essentially absent in the single molecule reorientations. Good agreement is found in the self‐diffusion coefficient over the entire temperature range.

Finite-Q cavity electrodynamics: dynamical and statistical aspects

Abstract: Solutions of the basic equations of a simplified model of the cavity quantum electrodynamics are presented under the condition that the single-photon Rabi frequency is much larger than the cavity decay rate. Such solutions are used to calculate the quantum-statistical properties of the field and atomic observables under a variety of initial conditions involving the states of the field and atom. Effects of increasing cavity damping and of the addition of thermal photons on collapse and revival phenomena are discussed. Phase-sensitive aspects of the cavity field are also treated. Quantum effects are shown to be most easily isolated by sending an atom initially prepared in a dressed state for such a state does not evolve further under the influence of a classical field. The appearance of squeezing in the cavity field is demonstrated. The squeezing is most prominent for a coherently prepared atom passing through an empty cavity. The quantum features of the complex dipole moment and their detectability are also discussed in detail.

Centered and eccentric geomagnetic dipoles and their poles, 1600–1985

Abstract: Using a unified approach, expressions are derived for the various pole positions and other dipole parameters for the centered and eccentric dipole models of the earth's magnetic field. The pole positions and other parameters are then calculated using the 1945–1985 International Geomagnetic Reference Field Gauss coefficients and coefficients from models of the earth's field for earlier epochs. Comparison is made between (1) the recent pole positions and those pertaining since 1600 and (2) the various theoretical pole positions and the observed dip pole positions.

Velocity-dependent property surfaces and the theory of vibrational circular dichroism

Abstract: The familiar idea of a molecular property surface in which a property (such as the dipole moment) is expressed as a function of nuclear coordinates R I , is extended to cover properties odd under time-reversal by including the nuclear velocities ·R I as variables. One such property is the magnetic dipole moment m . This approach relates magnetic dipole vibrational transition moments to derivatives (∂ m / ∂ ·R I ) which may be calculated within the clamped-nucleus approximation, and used in the theory of vibrational circular dichroism. The same derivatives describe the electric field at a nucleus induced by an external dynamic magnetic field, leading to a physically transparent description of VCD. Sum rules relate these derivatives to the molecular paramagnetizability.

s and d Rydberg series of NO probed by double resonance multiphoton ionization

Abstract: The s and d Rydberg states of NO with v = 0 have been observed for 6 leqslant; n* leqslant; 40 by double resonance multiphoton ionization which was three-photon resonant with the 3pπC 2∏ (v = 0) Rydberg intermediate state. Strong parity selection is shown to occur in the intermediate level despite the fact that the Λ-doublets are not resolved. The OODR spectra are thereby considerably simplified. A unified description of the rotational-electronic structure of the s and d Rydberg states in terms of quantum defect theory is presented, extending from the low n* s, d supercomplexes up beyond the ionization threshold. Electronic radial dipole transition moments are calculated and included in the MQDT analysis, giving evidence for a Cooper minimum in the region of the supercomplex n* = 5. For 25 leqslant; n* leqslant; 40 fringes are observed in the OODR spectrum and are interpreted in terms of rotational l-uncoupling, electronic s ∼d mixing and energy dependent electronic dipole transition moments. The fringe f...

The magnetic field of Uranus

Abstract: Aspherical harmonic model of the planetary magnetic field of Uranus is obtained from the Voyager 2 encounter observations using generalized inverse techniques which allow partial solutions to complex (underdetermined) problems. The Goddard Space Flight Center 'Q3' model is characterized by a large dipole tilt (58.6 deg) relative to the rotation axis, a dipole moment of 0.228 G R(Uranus radii cubed) and an unusually large quadrupole moment. Characteristics of this complex model magnetic field are illustrated using contour maps of the field on the planet's surface and discussed in the context of possible dynamo generation in the relatively poorly conducting 'ice' mantle.

Magnitude and direction of the change in dipole moment associated with excitation of the primary electron donor in Rhodopseudomonas sphaeroides reaction centers

Abstract: The magnitude and direction of the change in dipole moment, ..delta mu.., associated with the Q/sub y/ transition of the dimeric primary electron donor (special pair or P870) in Rhodopseudomonas sphaeroides reaction centers have been measured by Stark spectroscopy at 20 /sup 0/C. The magnitude of ..delta mu.. is found to be f/sup -1/ (10.3 +/- 0.7) D, where f is a correction factor for the local dielectric properties of the protein matrix. With the spherical cavity approximation and an effective local dielectric constant of 2, f = 1.2, and absolute value of ..delta mu.. is 8.6 +/- 0.6 D. Absolute value of ..delta mu.. for the Q/sub y/ transition of the special pair is approximately a factor of 3.4 and 2 greater than for the monomeric bacteriochlorophylls and bacteriopheophytins, respectively, in the reaction center. The angle between ..delta mu.. and the transition dipole moment for excitation of the first singlet electron state of the special pair was found to be 24 +/- 2/sup 0/. The measured values are combined to suggest a physical model in which the lowest excited singlet state of the special pair has substantial charge-transfer character and where charge is separated between the two monomers comprisingmore » the dimeric special pair. This leads to the hypothesis that the first charge-separated state in bacterial photosynthesis is formed directly upon photoexcitation. These data provide stringent values for comparison with theoretical calculations of the electronic structure of the chromophores in the reaction center.« less

Dissociation of NH3 to NH2+H

Abstract: Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet X and A states of NH3 yielding NH2 (X 2B1,A 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The A state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 A (in the NH2 fragment), and RNH =1.323 A (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out‐of‐plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the X–A conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the X and A states in planar structures depends strong...