Dipole Oscillator Strength Distributions Research Articles

Directional Dependence of the Mean Excitation Energy and Spectral Moments of the Dipole Oscillator Strength Distribution of Glycine and Its Zwitterion

In this contribution, we consider the interaction of glycine, a small, model biomolecule, and its zwitterion with fast ion radiation. The object of the study is to determine the differences in properties among various conformers and orientations of the neutral molecule and the zwitterion and to determine if these differences will have implications in terms of radiation protection and radiation therapy. To this end, quantum mechanical calculations were carried out on three conformers of the neutral molecule and two of the zwitterion to determine both the isotropic and directional components of the moments of the dipole oscillator strength distribution in each case. It is these moments that determine the interaction of swift radiation with a molecule.

Energy Ranking of Molecular Crystals Using Density Functional Theory Calculations and an Empirical van der Waals Correction

By combination of high level density functional theory (DFT) calculations with an empirical van der Waals correction, a hybrid method has been designed and parametrized that provides unprecedented accuracy for the structure optimization and the energy ranking of molecular crystals. All DFT calculations are carried out using the VASP program. The van der Waals correction is expressed as the sum over atom-atom pair potentials with each pair potential for two atoms A and B being the product of an asymptotic C(6,A,B)/r(6) term and a damping function d(A,B)(r). Empirical parameters are provided for the elements H, C, N, O, F, Cl, and S. Following Wu and Yang, the C(6) coefficients have been determined by least-squares fitting to molecular C(6) coefficients derived by Meath and co-workers from dipole oscillator strength distributions. The damping functions d(A,B)(r) guarantee the crossover from the asymptotic C(6,A,B)/r(6) behavior at large interatomic distances to a constant interaction energy at short distances. The careful parametrization of the damping functions is of crucial importance to obtain the correct balance between the DFT part of the lattice energy and the contribution from the empirical van der Waals correction. The damping functions have been adjusted to yield the best possible agreement between the unit cells of a set of experimental low temperature crystal structures and their counterparts obtained by lattice energy optimization using the hybrid method. On average, the experimental and the calculated unit cell lengths deviate by 1%. To assess the performance of the hybrid method with respect to the lattice energy ranking of molecular crystals, various crystal packings of ethane, ethylene, acetylene, methanol, acetic acid, and urea have been generated with Accelrys' Polymorph Predictor in a first step and optimized with the hybrid method in a second step. In five out of six cases, the experimentally observed low-temperature crystal structure corresponds to the most stable calculated structure.

Dipole Oscillator Strength Distributions and Properties for Methanol, Ethanol and Propan-1-ol and Related Dispersion Energies

Recommended isotropic dipole oscillator strength distributions (DOSDs) have been constructed for the methanol and ethanol molecules through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength (DOS) data; the DOS data employed are recent experimental results not available at the time of the original constrained DOSD analysis of these molecules. The constraints are furnished by molar refractivity data and the Thomas-Reiche-Kuhn sum rule. The DOSDs are used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for the molecules. Pseudo-DOSDs for these molecules, and for propan-1-ol based on an earlier constrained DOSD analysis for this molecule, are also presented. They are used to obtain reliable results for the isotropic dipole-dipole dispersion energy coefficients C6, for the interactions of the alcohols with each other and with 36 other species, and the triple-dipole dispersion energy coefficients C9for interactions involving any triple of molecules involving methanol, ethanol and propan-1-ol.

Reliable results for the Isotropic Dipole – Dipole and Triple – Dipole Dispersion Energy Coefficients for Interactions involving Formaldehyde, Acetaldehyde, Acetone, and Mono - , Di - , and Tri - Methylamine

Pseudo - spectral dipole oscillator strengths and excitation energies, which are discrete representations of previously developed recommended continuous dipole oscillator strength distributions(DOSDs), are presented for the ground state formaldehyde, acetaldehyde, acetone, and mono - , di - , and tri - methylamine molecules. These pseudo - DOSDs, together with previously published pseudo - DOSDs for other atoms and molecules, are used to evaluate the dipole - dipole and the triple - dipole dispersion energy coefficients for all the two - body and three - body interactions between H$_2$CO, CH$_3$CHO, (CH$_3$)$_2$CO, CH$_3$NH$_2$, (CH$_3$)$_2$ NH, and (CH$_3$)$_3$N, and between these molecules and forty - four other species, namely Cl$_2$, SiH$_4$, SiF$_4$, CCl$_4$, H, Li, He, Ne, Ar, Kr, Xe, SF6, HF, HCl, HBr, SO$_2$, CS$_2$, OCS, H$_2$, N$_2$, O$_2$, NO, N$_2$O, H$_2$O, H$_2$S, NH$_3$, CO, CO$_2$, the normal alkanes CH$_4$, C$_2$H$_6$, C$_3$H$_8$, C$_4$H$_{10}$, C$_5$H$_{12}$, C$_6$H$_{14}$, C$_7$H$_{16}$ and C$_8$H$_{18}$, the 1-alkenes C$_2$H$_4$, C$_3$H$_6$ and C$_4$H$_8$, C$_2$H$_2$, C$_6$H$_6$, and the primary alcohols CH$_3$OH, C$_2$H$_5$OH and C$_3$H$_7$OH. Results are presented explicitly for all the dipole - dipole dispersion energy coefficients and for the triple - dipole coefficients for all three -body interactions involving the H$_2$CO, CH$_3$CHO, (CH$_3$)$_2$CO, CH$_3$NH$_2$, (CH$_3$)$_2$ NH, and (CH$_3$)$_3$N molecules. The estimated errors in the two - body and three - coefficients are 1% and 1-2%

New approximations for calculating dispersion coefficients.

Improved approaches for finding approximate values of dispersion coefficients are proposed. They are based on scaling the values of time-dependent Hartree-Fock (TDHF) dispersion coefficients by factors that use the ratio of the estimated true value and the TDHF value of static dipole polarizabilities. It is shown that for a set of 14 atoms and molecules the average absolute-value deviation of the estimated two-body isotropic dispersion coefficients with respect to the dipole oscillator strength distribution results is smaller than 1.0% for two of our approaches. For three-body isotropic dispersion coefficients the corresponding deviations are smaller than 1.2%. Our approximations work particularly well compared to approximations proposed by other researchers in cases where the TDHF results differ from the reference values by more than 10%.

Dipole oscillator strengths, dipole properties and dispersion energies for SiF4

A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the silicon tetrafluoride (SiF4) molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength data. The constraints are furnished by experimental molar refractivity data and the Thomas-Reiche-Kuhn sum rule. The DOSD is used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums and mean excitation energies for the molecule. A pseudo-DOSD for SiF4 is also presented which is used to obtain reliable results for the isotropic dipole-dipole dispersion energy coefficients C6, for the interaction of SiF4 with itself and with 43 other species and the triple-dipole dispersion energy coefficient C9 for (SiF4)3.

Dipole oscillator strength properties and dispersion energies for SiH4

A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the silane (SiH4) molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength data. The constraints are furnished by experimental molar refractivity data and the Thomas–Reiche–Kuhn sum rule. The DOSD is used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for the molecule. A pseudo-DOSD for SiH4 is also presented which is used to obtain reliable results for the isotropic dipole–dipole dispersion energy coefficients C6, for the interaction of silane with itself and with forty-four other species, and the triple–dipole dispersion energy coefficient C9 for ðSiH4Þ 3 . 2002 Elsevier Science B.V. All rights reserved.

Dipole oscillator strength properties and dispersion energies for CI2

A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the chlorine molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength and molar refractivity data. It has been used to evaluate a variety of dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for the molecule. A pseudo-DOSD for C12 is also presented which is used to obtain reliable results for the isotropic dipole-dipole dispersion energy coefficients C6, for the interaction of Cl2 with itself and forty-two other species, and the triple-dipole dispersion energy coefficient C9 for (Cl2)3.

Reliable isotropic dipole properties and dispersion energy coefficients for CCl 4

Dipole oscillator strength distributions (DOSD) have been constructed for ground state CCl 4 molecule, by using constrained least-squares methods, experimental and mixture rule dipole oscillator strength data, and refractive index measurements. A recommended isotropic DOSD is used to evaluate a wide variety of dipole oscillator strength sums S K , logarithmic dipole sums L K and mean excitation energies I K . Pseudo-DOSDs have been constructed for CCl 4 molecule and these are used to calculate isotropic dipole–dipole dispersion energy coefficients C 6 for the interaction of CCl 4 with itself and with 45 other species, and the triple-dipole dispersion energy coefficients C 9 for (CCl 4) 3.

Ionization and excitation yields in liquid water due to the primary irradiation: Relationship of radiolysis with far UV-photolysisPresented at the Symposium on Recent Trends in Photochemical Sciences, Trivandrum, January 8-10, 2000.

First collision yields of ionization and dissociation in liquid water, due to incident electrons of energy 500 eV to 1 MeV, have been calculated in the optical approximation by using the experimental dipole oscillator strength distribution (DOSD) and the recently determined photoionization and photodissociation quantum efficiencies in the far UV. From these the G value of ionization for the complete absorption of these radiations is estimated employing a heuristic method based on energy balance. It is found that ca. 25% of total ionization and <5% of total dissociation originate from primary interactions alone; the rest are due to the action of the secondary electrons. Incorporation of dissociative electron attachment (DEA) is necessary to account for the total radiolytic dissociation yield. A new definition of ionization potential in liquid water is provided depending on the mechanism and a tentative value of 11.7 eV is assigned to it.

Energy loss by non-relativistic electrons and positrons in liquid water

Inelastic collision cross-sections, mean free paths, stopping powers, energy loss distributions, mean energy losses and csda ranges are evaluated for non-relativistic electrons and positrons in liquid water using a formalism in which the response of the medium is expressed employing the experimental dipole oscillator strength distribution. Monte Carlo track structure simulations employing the calculated inelastic collision cross-sections, and elastic cross-sections evaluated using partial wave methods, are used to determine the energy dependence of positron and electron path-lengths, penetrations, and non-homogeneous energy deposition distributions. The calculated data are discussed, and compared and contrasted. The energy loss properties of electrons and positrons that depend only on the differential inelastic collision cross-section are similar for particle energies ≳1 keV, but there are apparent differences for lower energies. The energy loss properties depending on the size of the energy transfer events and on the differential inelastic collision cross-section differ for all particle energies. The differences found are the consequence of electron indistinguishability on the inelastic cross-section for the electron. Non-homogeneous energy deposition distributions of positrons with energy less than 1 keV are significantly more forward directed than those of electrons with the same initial energy. The differences are due to the larger inelastic collision cross-section of a positron compared to an electron, and its effect on the relative numbers of inelastic and elastic collisions.

Empirical correction to density functional theory for van der Waals interactions

An empirical method has been designed to account for the van der Waals interactions in practical molecular calculations with density functional theory. For each atom pair separated at a distance R, the method adds to the density functional electronic structure calculations an additional attraction energy EvdW=−fd(R)C6R−6, where fd(R) is the damping function which equals to one at large value of R and zero at small value of R. The coefficients C6 for pair interactions between hydrogen, carbon, nitrogen, and oxygen atoms have been developed in this work by a least-square fitting to the molecular C6 coefficients obtained from the dipole oscillator strength distribution method by Meath and co-workers. Two forms of the damping functions have been studied, with one dropping to zero at short distances much faster than the other. Four density functionals have been examined: Becke’s three parameter hybrid functional with the Lee-Yang-Parr correlation functional, Becke’s 1988 exchange functional with the LYP correlation functional, Becke’s 1988 exchange functional with Perdew and Wang’s 1991 (PW91) correlation functional, and PW91 exchange and correlation functional. The method has been applied to three systems where the van der Waals attractions are known to be important: rare-gas diatomic molecules, stacking of base pairs and polyalanines’ conformation stabilities. The results show that this empirical method, with the damping function dropping to zero smoothly, provides a significant correction to both of the Becke’s hybrid functional and the PW91 exchange and correlation functional. Results are comparable to the corresponding second-order Møller-Plesset calculations in many cases.

Energy Loss by Nonrelativistic Electrons and Positrons in Polymers and Simple Solid Hydrocarbons

The energy loss and transport of electrons and positrons in gaseous cyclohexane, and solid cyclohexane, benzene, polyethylene, and polystyrene are determined and discussed. A formalism for the energy loss properties of nonrelativistic positrons, analogous to that previously developed for electrons, is presented. This approach makes use of the dipole oscillator strength distributions of the medium, which are constructed here for polyethylene and polystyrene from their optical properties and from atomic X-ray photoabsorption coefficients. Inelastic collision cross-sections, mean free paths, stopping powers, energy loss distributions, mean energy losses, and csda ranges are evaluated for electrons in gaseous and solid cyclohexane, solid benzene, polyethylene, and polystyrene. Those properties depending on the zeroth energy moment of the inelastic collision cross-section are similar for the solid media, while those depending on the first energy moment show significant differences. The data for electrons are discussed and compared and contrasted with the results of similar calculations for positrons. For energies greater than ∼1 keV, the energy loss properties of electrons and positrons are similar, but for lower energies there are very apparent differences.

Calculation of inelastic cross-sections for the interaction of electrons with amorphous selenium

Research into medical x-ray detectors based on amorphous selenium (a-Se) has seen a resurgence lately due to the advent of flat-panel active matrix technology. There is presently no quantitative theory, however, to explain the signal formation due to the interaction of ionizing radiation with a-Se. One of the main obstacles in the characterization of this process is the lack of suitable interaction cross-sections. In this work we develop an expression for the calculation of inelastic electron cross-sections from a dipole oscillator strength distribution, which includes relativistic and exchange effects. We construct the oscillator strength distribution from known experimental results and calculate the total and cumulative differential cross-sections for incident electron energies ranging from a few eV to 23 MeV. The total cross-section agrees with that of isolated Se atoms at high energies, as expected, and provides the low energy cross-sections which depend on the dielectric properties of the amorphous state. From these cross-sections, Monte Carlo codes can be developed which simulate the creation of individual electron-hole pairs, which will lead to a better understanding of the signal-formation process in a-Se based x-ray detectors.

Ab initio dynamic dipole polarizabilities for O2, its photoabsorption spectrum in the Schumann–Runge region, and long-range interaction coefficients for its dimer

Perturbed states representing the response to an external electric field are obtained in the multireference averaged coupled pair functional formalism based on orbitals which are optimized in a perturbed multireference self-consistent field procedure. For each perturbing operator perturbed wave functions for several frequencies of the perturbing field are obtained simultaneously. From these sets of perturbed states reduced spectra are derived which represent effective oscillator strength distributions. The broad shape of the Schumann–Runge continuum leads to a poor description of the dynamic polarizability for frequencies approaching the first pole when only vertical transitions are considered. To account for this effect the absorption spectrum in the Schumann–Runge region is calculated and the polarizability is described up to the range of anomalous dispersion. The dynamic polarizabilities and the interaction coefficients are found to be in excellent agreement with experimental data where it is available. Significant differencies of up to 20% are observed between our anisotropic interaction coefficients and earlier estimates obtained from semiempirical anisotropic dipole oscillator strength distributions.

Collision-induced shift of the ionization continuum and interaction polarizabilities of rare-gas atoms

Contributions to the incremental pair polarizability traces of Ne, Ar, Kr and Xe due to the shift of the ionization continua are evaluated using the Kramers–Kronig-type integral expression modified to account for the effect of atomic collisions. Interaction trace functions are computed and their shapes explained by the interplay between attractive and repulsive interactions, resulting in the redistribution of the dipole oscillator strength distribution in the continuum. The effect of the spin–orbit coupling on the interaction polarizability is demonstrated. The calculated second dielectric virial coefficients B ε are in better agreement with the room-temperature experimental data than the results of other recent studies are. The strong temperature dependence of B ε found in experiments remains unexplained, however.

Collision-induced shift of the ionization continuum and interaction polarizabilities of rare-gas atoms

Contributions to the incremental pair polarizability traces of Ne, Ar, Kr and Xe due to the shift of the ionization continua are evaluated using the Kramers–Kronig-type integral expression modified to account for the effect of atomic collisions. Interaction trace functions are computed and their shapes explained by the interplay between attractive and repulsive interactions, resulting in the redistribution of the dipole oscillator strength distribution in the continuum. The effect of the spin–orbit coupling on the interaction polarizability is demonstrated. The calculated second dielectric virial coefficients B ε are in better agreement with the room-temperature experimental data than the results of other recent studies are. The strong temperature dependence of B ε found in experiments remains unexplained, however.

Isotropic dipole properties for acetone, acetaldehyde and formaldehyde

Dipole oscillator strength distributions have been constructed for acetone, acetaldehyde and formaldehyde, by using constrained least-squares methods, experimental and mixture rule dipole oscillator strength data, and molar refractivity data. The distributions have been used to evaluate recommended values for a wide variety of isotropic dipole oscillator strength sums, logarithmic dipole sums, and mean excitation energies for each molecule. Often these results are the only (reliable) values available for many of these properties which are of importance in a variety of research areas.

Isotropic dipole properties for acetone, acetaldehyde and formaldehyde

Dipole oscillator strength distributions have been constructed for acetone, acetaldehyde and formaldehyde, by using constrained least-squares methods, experimental and mixture rule dipole oscillator strength data, and molar refractivity data. The distributions have been used to evaluate recommended values for a wide variety of isotropic dipole oscillator strength sums, logarithmic dipole sums, and mean excitation energies for each molecule. Often these results are the only (reliable) values available for many of these properties which are of importance in a variety of research areas.

Calculations of electronic energy loss of fast molecules in amorphous carbon

The energy loss of fast H 2 + in amorphous carbon has been calculated within the first Born approximation. We used two models: a simple Bethe-like estimate and an oscillator model. The basic input needed for both models is the dipole oscillator strength distribution for the target material. For amorphous carbon we applied a well etablished empirical set based on optical data. We find that the predictions compare well with each other, in particular that the shell corrections do not influence the proximity effect. The theoretical results are compared with experimental data.