Exchange Corrections Research Articles

Two-photon exchange contribution to elastic e− -proton scattering: Full dispersive treatment of πN states and comparison with data

We evaluate the two-photon exchange correction to the elastic electron-proton scattering cross section within a dispersive framework. Besides the elastic contribution, we account for all $\pi N$ intermediate state contributions using the phenomenological MAID fit as an input. We develop a novel method for the analytical continuation of the two-photon exchange amplitudes into the unphysical region and generalize our previous work to the momentum transfer region $ 0.064~\mathrm{GeV}^2 \lesssim Q^2 \lesssim 1~\mathrm{GeV}^2$. We compare our results with recent OLYMPUS, CLAS and VEPP-3 data as well as with empirical fits and estimates in the forward angular region.

Influence of a Gas Exchange Correction Procedure on Resting Metabolic Rate and Respiratory Quotient in Humans.

The aim of this study was to determine the influence of a gas exchange correction protocol on resting metabolic rate (RMR) and respiratory quotient (RQ), assessed by a Vmax Encore 29n metabolic cart (SensorMedics Co., Yorba Linda, California) in overnight fasted and fed humans, and to assess the predictive power of body size for corrected and uncorrected RMR. Healthy participants (23 M/29 F; 34 ± 9 years old; 26.3 ± 3.7 kg/m2 ) ingested two 3-hour-apart glucose loads (75 g). Indirect calorimetry was conducted before and hourly over a 6-hour period. Immediately after indirect calorimetry assessment, gas exchange was simulated through high-precision mass-flow regulators, which permitted the correction of RMR and RQ values. Uncorrected and corrected RMR and RQ were directly related at each time over the 6-hour period. However, uncorrected versus corrected RMR was 6.9% ± 0.5% higher (128 ± 7 kcal/d; P < 0.0001), while RQ was 14.0 ± 0.4% lower (-0.114 ± 0.003; P < 0.0001) when compared throughout the whole period. Body weight, sex, and age explained a larger fraction of the variance when corrected RMR was considered (adjusted R2 = 0.71; P < 0.0001) versus uncorrected RMR (adjusted R2 = 0.59; P < 0.0001). Applying a protocol to correct gas exchange in humans over a 6-hour period is feasible and provides information of improved accuracy.

Empirical Correction for Differences in Chemical Exchange Rates in Hydrogen Exchange-Mass Spectrometry Measurements.

A barrier to the use of hydrogen exchange-mass spectrometry (HX-MS) in many contexts, especially analytical characterization of various protein therapeutic candidates, is that differences in temperature, pH, ionic strength, buffering agent, or other additives can alter chemical exchange rates, making HX data gathered under differing solution conditions difficult to compare. Here, we present data demonstrating that HX chemical exchange rates can be substantially altered not only by the well-established variables of temperature and pH but also by additives including arginine, guanidine, methionine, and thiocyanate. To compensate for these additive effects, we have developed an empirical method to correct the hydrogen-exchange data for these differences. First, differences in chemical exchange rates are measured by use of an unstructured reporter peptide, YPI. An empirical chemical exchange correction factor, determined by use of the HX data from the reporter peptide, is then applied to the HX measurements obtained from a protein of interest under different solution conditions. We demonstrate that the correction is experimentally sound through simulation and in a proof-of-concept experiment using unstructured peptides under slow-exchange conditions (pD 4.5 at ambient temperature). To illustrate its utility, we applied the correction to HX-MS excipient screening data collected for a pharmaceutically relevant IgG4 mAb being characterized to determine the effects of different formulations on backbone dynamics.

Dispersive approach to two-photon exchange in elastic electron-proton scattering

We examine the two-photon exchange corrections to elastic electron-proton scattering within a dispersive approach, including contributions from both Nucleon and Delta intermediate states. The dispersive analysis avoids off-shell uncertainties inherent in traditional approaches based on direct evaluation of loop diagrams, and guarantees the correct unitary behavior in the high energy limit. Using empirical information on the electromagnetic nucleon elastic and N-Delta transition form factors, we compute the two-photon exchange corrections both algebraically and numerically. Results are compared with recent measurements of e+p to e-p cross section ratios from the CLAS, VEPP-3 and OLYMPUS experiments, as well as with polarization transfer observables.

Prediction of e± elastic scattering cross-section ratio based on phenomenological two-photon exchange corrections

Prediction of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>e</mml:mi><mml:mo>±</mml:mo></mml:msup></mml:math> elastic scattering cross-section ratio based on phenomenological two-photon exchange corrections

Physical properties of the spin Hamiltonian on honeycomb lattice samples with Kekulé and vacuum polarization corrections

Magnetic and thermodynamical properties of a system of spins in a honeycomb lattice, such as magnetization, magnetic susceptibility and specific heat, in a low-temperature regime are investigated by considering the effects of a Kekulé scalar exchange and QED vacuum polarization corrections to the interparticle potential. The spin lattice calculations are carried out by means of Monte Carlo simulations. We present a number of comparative plots of all the physical quantities we have considered and a detailed analysis is presented to illustrate the main features and the variation profiles of the properties with the applied external magnetic field and temperature.

Assessing the role of Hartree-Fock exchange, correlation energy and long range corrections in evaluating ionization potential, and electron affinity in density functional theory.

Accurate determination of ionization potentials (IPs), electron affinities (EAs), fundamental gaps (FGs), and HOMO, LUMO energy levels of organic molecules play an important role in modeling and predicting the efficiencies of organic photovoltaics, OLEDs etc. In this work, we investigate the effects of Hartree Fock (HF) Exchange, correlation energy, and long range corrections in predicting IP and EA in Hybrid Functionals. We observe increase in percentage of HF exchange results in increase of IPs and decrease in EAs. Contrary to the general expectations inclusion of both HF exchange and correlation energy (from the second order perturbation theory MP2) leads to poor prediction. Range separated Hybrid Functionals are found to be more reliable among various DFT Functionals investigated. DFT Functionals predict accurate IPs whereas post HF methods predict accurate EAs. © 2017 Wiley Periodicals, Inc.

Two-photon exchange corrections to elastic e− -proton scattering: Full dispersive treatment of πN states at low momentum transfers

We evaluate the pion-nucleon intermediate-state contribution to the two-photon exchange (TPE) correction in the elastic electron-nucleon scattering within a dispersive framework. We calculate the contribution from all $\pi N$ partial waves using the MAID parametrization. We provide the corresponding TPE correction to the unpolarized $e p$ scattering cross section in the region of low momentum transfer $ Q^2 \lesssim 0.064~\mathrm{GeV}^2$, where no analytical continuation into the unphysical region of the TPE scattering amplitudes is required. We compare our result in the forward angular region with an alternative TPE calculation, in terms of structure functions, and find a good agreement, indicating a small contribution at low $Q^2$ due to discontinuities beyond $ \pi N$. We also compare our results with empirical fits.

The role of the long-range exchange corrections in the description of electron delocalization in aromatic species.

In this article, we address the role of the long-range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange-correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long-standing debate in the literature. The presented results indicate that the noncorrected exchange-correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteria. © 2017 Wiley Periodicals, Inc.

Probing the diffractive production of aZ-boson pair at forward rapidities at the LHC

In this paper, we present the results from phenomenological analysis of Z boson pair hard diffractive production at the LHC. The calculation is based on the Regge factorization approach. Diffractive parton density functions extracted by the H1 Collaboration at DESY-HERA are used. The multiple Pomeron exchange corrections are considered through the rapidity gap survival probability factor. We give numerical predictions for single diffractive as well as double Pomeron exchange cross sections and compare with the photon-induced and non-diffractive ones. The contributions from quark-anti-quark collision and gluon-gluon fusion are displayed. Various kinematical distributions are presented. We make predictions which could be compared to future measurements at the LHC where forward proton detectors are installed and detector acceptances are considered.

Two-photon exchange correction to2S−2Psplitting in muonicHe3ions

We calculate the two-photon exchange correction to the Lamb shift in muonic helium-3 ions within the dispersion relations framework. Part of the effort entailed making analytic fits to the electron-$^3$He quasielastic scattering data set, for purposes of doing the dispersion integrals. Our result is that the energy of the 2$S$ state is shifted downwards by two-photon exchange effects by 15.14(49) meV, in good accord with the result obtained from a potential model and effective field theory calculation.

Quantum kinetic theories in degenerate plasmas

In this review we give an overview of the recent work on quantum kinetic theories of plasmas. We focus, in particular, on the case where the electrons are fully degenerate. For such systems, perturbation methods using the distribution function can be problematic. Instead we present a model that considers the dynamics of the Fermi surface. The advantage of this model is that, even though the value of the distribution function can be greatly perturbed outside the equilibrium Fermi surface, deformation of the Fermi surface is small up to very large amplitudes. Next, we investigate the short-scale dynamics for which the Wigner–Moyal equation replaces the Vlasov equation. In particular, we study wave–particle interaction, and deduce that new types of wave damping can occur due to the simultaneous absorption (or emission) of multiple wave quanta. Finally, we consider exchange effects within a quantum kinetic formalism to find a model that is more accurate than those using exchange potentials from density functional theory. We deduce the exchange corrections to the dispersion relations for Langmuir and ion-acoustic waves. In comparison to results based on exchange potentials deduced from density functional theory we find that the latter models are reasonably accurate for Langmuir waves, but rather inaccurate for ion acoustic waves.

Shock compressibility of iron calculated in the framework of quantum-statistical models with different ionic parts

Quantum-statistical calculations of shock compressibility of iron are performed. Electronic part of thermodynamic functions is calculated in the framework of three quantum-statistical approaches: the Thomas-Fermi, the Thomas-Fermi with quantum and exchange corrections and the Hartree-Fock-Slater models. The influence of ionic part of thermodynamic functions is taken into account separately with using three models: the ideal gas, the one-component plasma and the charged hard spheres models. The results of calculations are presented in the pressure range from 1 to 107 GPa for samples with initially densities 7.85, 4.31 and 2.27 g/cm3. Calculated Hugoniots are compared with available experimental data.

Region of validity of the Thomas–Fermi model with corrections

A new method to calculate thermodynamically consistent shell corrections in a wide range of parameters is used to predict the region of validity of the Thomas-Fermi approach. The method is applicable both at low and high density. Thermodynamic functions of electrons calculated by the Thomas–Fermi model are compared with quantum, exchange, and shell corrections. The corrections become quite big at moderate and low densities and low temperatures in the region of strongly coupled plasma.

Region of validity of the Thomas–Fermi model with quantum, exchange and shell corrections

A novel approach to calculate thermodynamically consistent shell corrections in wide range of parameters is used to predict the region of validity of the Thomas-Fermi approach. Calculated thermodynamic functions of electrons at high density are consistent with the more precise density functional theory. It makes it possible to work out a semi-classical model applicable both at low and high density.

Ab initio study of thermodynamically consistent equation of state of warm dense aluminum plasma

Thermodynamically consistent equation of state (EOS) of two-temperature aluminum across a wide range of parameter space (compression ratio ratios V0/V up to 4, electronic temperatures Te up to 1 500 000 K, and ionic temperature Tion up to 10 000 000 K for Te up to 40 000 K) is investigated from the free energy calculations using density functional theory (DFT) based first-principles electronic structure methods. Our results can serve as a stringent benchmark for the present EOS model and database, where various approximations are adopted, used in hydrodynamic simulations as well as developing new EOS models. We find that the Thomas-Fermi model for the electronic pressure overestimates the EOS within the present parameter space, whereas the Thomas-Fermi model with exchange corrections are in good agreement with our results for Te greater than 600 000 K. The ionic pressure for a given ionic temperature Tion is found to be nearly independent of the electronic temperature at high temperatures, which can be modeled with kinetic theory for Tion larger than 1 000 000 K for various Te. The asymptotic behavior of the electronic contributions to the plasma pressure is further analyzed and casted into a compact analytical form with a few fitting parameters. This analytical form is physically well motivated and reproduces the desired asymptotic behaviors of the EOS within the interested parameter space. Therefore, our results can be conveniently used for modeling important properties and processes of high energy density systems.

Carbon-hydrogen bond activation by a titanium neopentylidene complex

The titanium neopentylidene complex (PNP)Ti=CHtBu(CH2tBu), PNP=N[2-PiPr2-4-methylphenyl]2−, can activate both sp2 and sp3 C–H bonds under mild conditions. In this work, we studied the reaction mechanism of this complex with benzene and methane using modern density functional theory, specifically the ωB97XD functional which contains long-range exchange and dispersion corrections. The mechanism of the reaction is similar to that computed previously in the literature, but we describe a new conformer that is both more stable and kinetically more reactive. The four-step mechanism is very similar for both benzene and methane. However, the highest energy barriers differ; for methane, it is the last step, which elucidates the inertness of that reactant. In addition, the hydrogen exchange between alkyl and alkylidene ligands in methane’s product was studied by two different mechanisms: tautomerization to form (PNP)TiCHtBu(=CH2) and reverse C–H activation to form (PNP)Ti≡CtBu(CH3). The feasibility of the tautomerization, through a preliminary, accessible isomerization, suggests that these systems can be used to explore the reactivity of terminal methylidenes. Finally, methodological considerations are also discussed, as the importance of including the dispersion in the density functionals was determined by comparing several functionals. This comparison has shown that the dispersion is critical for accurate modeling, especially in the stability of the unsaturated intermediate; this has been neglected in previous studies.

Two-photon exchange correction to muon–proton elastic scattering at low momentum transfer

We evaluate the two-photon exchange (TPE) correction to the muon-proton elastic scattering at small momentum transfer. Besides the elastic (nucleon) intermediate state contribution, which is calculated exactly, we account for the inelastic intermediate states by expressing the TPE process approximately through the forward doubly virtual Compton scattering. The input in our evaluation is given by the unpolarized proton structure functions and by one subtraction function. For the latter, we provide an explicit evaluation based on a Regge fit of high-energy proton structure function data. It is found that, for the kinematics of the forthcoming muon-proton elastic scattering data of the MUSE experiment, the elastic TPE contribution dominates, and the size of the inelastic TPE contributions is within the anticipated error of the forthcoming data.

Two-photon exchange correction in elastic unpolarized electron-proton scattering at small momentum transfer

We evaluate the two-photon exchange (TPE) correction to the unpolarized elastic electron-proton scattering at small momentum transfer $ Q^2 $. We account for the inelastic intermediate states approximating the double virtual Compton scattering by the unpolarized forward virtual Compton scattering. The unpolarized proton structure functions are used as input for the numerical evaluation of the inelastic contribution. Our calculation reproduces the leading terms in the $ Q^2 $ expansion of the TPE correction and goes beyond this approximation by keeping the full $ Q^2 $ dependence of the proton structure functions. In the range of small momentum transfer, our result is in good agreement with the empirical TPE fit to existing data.

Accurate Ionization Potentials and Electron Affinities of Acceptor Molecules III: A Benchmark of GW Methods

The performance of different GW methods is assessed for a set of 24 organic acceptors. Errors are evaluated with respect to coupled cluster singles, doubles, and perturbative triples [CCSD(T)] reference data for the vertical ionization potentials (IPs) and electron affinities (EAs), extrapolated to the complete basis set limit. Additional comparisons are made to experimental data, where available. We consider fully self-consistent GW (scGW), partial self-consistency in the Green's function (scGW0), non-self-consistent G0W0 based on several mean-field starting points, and a "beyond GW" second-order screened exchange (SOSEX) correction to G0W0. We also describe the implementation of the self-consistent Coulomb hole with screened exchange method (COHSEX), which serves as one of the mean-field starting points. The best performers overall are G0W0+SOSEX and G0W0 based on an IP-tuned long-range corrected hybrid functional with the former being more accurate for EAs and the latter for IPs. Both provide a balanced treatment of localized vs delocalized states and valence spectra in good agreement with photoemission spectroscopy (PES) experiments.