Substantial Cancellation Research Articles

Deciphering the Long-Distance Penguin Contribution to B[over ¯]_{d,s}→γγ Decays.

We compute for the first time the long-distance penguin contribution to the double radiative B-meson decays by applying the perturbative factorization theorem. The numerically dominant penguin amplitude arises from the soft-gluon radiation off the light up-quark loop rather than the counterpart charm-loop effect. Importantly, the long-distance up-quark penguin contribution brings about the substantial cancellation of the known factorizable power correction, thus enabling B_{d,s}→γγ to become new benchmark probes of physics beyond the standard model.

Molecular Geometries and Vibrational Contributions to Reaction Thermochemistry Are Surprisingly Insensitive to the Choice of Basis Sets.

Calculation of molecular geometries and harmonic vibrational frequencies are pre-requisites for thermochemistry calculations. Contrary to conventional wisdom, this paper demonstrates that quantum chemical predictions of the thermochemistry of many gas and solution phase chemical reactions appear to be very insensitive to the choice of basis sets. For a large test set of 80 diverse organic and transition-metal-containing reactions, variations in reaction free energy based on geometries and frequencies calculated using a variety of double and triple-zeta basis sets from the Pople, Jensen, Ahlrichs, and Dunning families are typically less than 4 kJ mol-1, especially when the quasiharmonic oscillator correction is applied to mitigate the effects of low-frequency modes. Our analysis indicates that for many organic molecules and their transition states, high-level revDSD-PBEP86-D4 and DLPNO-CCSD(T)/(aug-)cc-pVTZ single-point energies usually vary by less than 2 kJ mol-1 on density functional theory geometries optimized using basis sets ranging from 6-31+G(d) to aug-pcseg-2 and aug-cc-pVTZ. In cases where these single-point energies vary significantly, indicating sensitivity of molecular geometries to the choice of basis set, there is often substantial cancellation of errors when the reaction energy or barrier is calculated. The study concludes that the choice of basis set for molecular geometry and frequencies, particularly those considered in this study, is not critical for the accuracy of thermochemistry calculations in the gas or solution phase.

Time-dependent exchange-correlation hole and potential of the electron gas

The exchange-correlation hole and potential of the homogeneous electron gas have been investigated within the random-phase approximation, employing the plasmon-pole approximation for the linear density response function. The angular dependence as well as the time dependence of the exchange-correlation hole are illustrated for a Wigner-Seitz radius ${r}_{s}=4$ (atomic unit). It is found that there is a substantial cancellation between exchange and correlation potentials in space and time, analogous to the cancellation of exchange and correlation self-energies. Analysis of the sum rule explains why it is more advantageous to use a noninteracting Green function than a renormalized one when calculating the response function within the random-phase approximation and consequently the self-energy within the well-established $GW$ approximation. The present study provides a starting point for more accurate and comprehensive calculations of the exchange-correlation hole and potential of the electron gas with the aim of constructing a model based on the local density approximation as in density functional theory.

Student Debt Cancellation IS Progressive: Correcting Empirical and Conceptual Errors

We examine the argument that student loan cancellation would be a regressive policy and show how it arises from several empirical and conceptual errors. The authors conclude that student cancellation can, in fact, be progressive and that more substantial cancellation policies would be more progressive. The reason for this progressivity is simple: people from wealthy backgrounds (and their parents) rarely use student loans to pay for college. Through examination of five common mistakes that have led some to conclude that student debt cancellation is regressive, the authors show that student debt cancellation would provide more benefits to those with fewer economic resources and could play a critical role in addressing the racial wealth gap and building the Black middle class.

Challenges post-COVID-19: planning strategies to resume elective dentistry and allied surgical specialities

The effect of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic on the NHS in the UK has been profound and unprecedented. Many surgical specialities, including dentistry, throughout the country have not been exempt from this effect. As there are many aerosol-generating procedures and aerosol-generating exposures in surgical specialities, there has been a substantial cancellation of elective treatment. This has been in part because of the limited availability of personal protective equipment for surgeons as this is being use elsewhere by clinicians to aid the reduction of viral spread in the community. As the UK is preparing to emerge from the ‘lockdown’ during the pandemic, restarting elective surgical and dental treatment is an expected challenge. This article looks at the possible roadmap to recovery of elective surgical management and dentistry, taking into consideration possible predicted further peaks and troughs of COVID-19 infections.

Assessment of Approximate Methods for Anharmonic Free Energies.

Quantitative evaluation of the thermodynamic properties of materials-most notably their stability, as measured by the free energy-must take into account the role of thermal and zero-point energy fluctuations. While these effects can easily be estimated within a harmonic approximation, corrections arising from the anharmonic nature of the interatomic potential are often crucial and require computationally costly path integral simulations to obtain results that are essentially exact for a given potential. Consequently, different approximate frameworks for computing affordable estimates of the anharmonic free energies have been developed over the years. Understanding which of the approximations involved are justified for a given system, and therefore choosing the most suitable method, is complicated by the lack of comparative benchmarks. To facilitate this choice we assess the accuracy and efficiency of some of the most commonly used approximate methods: the independent mode framework, the vibrational self-consistent field, and self-consistent phonons. We compare the anharmonic correction to the Helmholtz free energy against reference path integral calculations. These benchmarks are performed for a diverse set of systems, ranging from simple weakly anharmonic solids to flexible molecular crystals with freely rotating units. The results suggest that, for simple solids such as allotropes of carbon, these methods yield results that are in excellent agreement with the reference calculations, at a considerably lower computational cost. For more complex molecular systems such as polymorphs of ice and paracetamol the methods do not consistently provide a reliable approximation of the anharmonic correction. Despite substantial cancellation of errors when comparing the stability of different phases, we do not observe a systematic improvement over the harmonic approximation even for relative free energies. We conclude that, at least for the classes of materials considered here, efforts toward obtaining computationally feasible anharmonic free energies should therefore be directed toward reducing the expense of path integral methods.

Predicting Octanol-Water Partition Coefficients: Are Quantum Mechanical Implicit Solvent Models Better than Empirical Fragment-Based Methods?

In this work, we examined the performance of contemporary quantum mechanical implicit solvent models (SMD, SM8, SM12, and ADF-COSMO-RS) and empirical fragment-based methods for predicting octanol-water partition coefficients (log Pow). Two test sets were chosen: the first is composed of 34 organic molecules from a recent study by Mobley J. Chem. Theory Comput , 2016 , 12 , 4015 - 4024 , and the second set is based on a collection of 55 fluorinated alkanols and carbohydrates from Linclau Angew. Chem., Int. Ed. , 2016 , 55 , 674 - 678 . Our analysis indicates that the errors in the solvation free energies of implicit models are reasonably systematic in both solvents such that there is substantial cancellation of errors in the calculation of transfer free energies. Overall, implicit solvent models performed very well across the two test sets with mean absolute errors (MAEs) of about 0.6 log unit and are superior to explicit solvent simulations (GAFF and GAFF-DC). Interestingly, the best performers were empirical fragment-based methods, including ALOGP and miLOGP with significantly lower MAEs (0.2 to 0.4 log unit). The ALOGP method was further tested against the recent SAMPL6 log Pow challenge consisting of 11 drug-like molecules where it obtained an MAE of 0.32 log unit compared to the best-performing COSMOtherm model (0.31 log unit).

Tuning the Permeability of Dense Membranes by Shaping Nanoscale Potentials.

Permeability is one of the most fundamental transport properties in soft matter physics, material engineering, and nanofluidics. Here, we report by means of Langevin simulations of ideal penetrants in a nanoscale membrane made of a fixed lattice of attractive interaction sites, how the permeability can be massively tuned, even minimized or maximized, by tailoring the potential energy landscape for the diffusing penetrants, depending on the membrane attraction, topology, and density. Supported by limiting scaling theories we demonstrate that the observed nonmonotonic behavior and the occurrence of extreme values of the permeability is far from trivial and triggered by a strong anticorrelation and substantial (orders of magnitude) cancellation between penetrant partitioning and diffusivity, especially within dense and highly attractive membranes.

Isotope dependence of the c 3Πu−b 3Σ and D 1Π−B′ 1Σ predissociation rates of molecular hydrogen

The state-specific predissociation rates of the (v,N,J) state by and the (v,J) state by the continuum of various isotopologues of molecular hydrogen have been calculated from accurate ab initio potential energy curves and electronic coupling matrix elements. Lifetimes and predissociation rates of the (v,N,J) and (v,J) levels and accurate energies of the (v,N) and (v,J) levels of the isotopologues have been obtained. Significant isotope dependence of state specific predissociation rate has been found even after adjustment for Franck-Condon factors and reduced mass. The use of average electronic matrix elements of H2 for other isotopologues underestimates the − predissociation rates of the HD, HT, D2, DT and T2 molecules by ∼12%, ∼16%, ∼30%, ∼40%, ∼52%, respectively, and the − rates of the HD, HT, D2, DT and T2 molecules by ∼10%, ∼15%, ∼26%, ∼35% and ∼45%, respectively. When compared at similar rotation, vibration and kinetic energies, the underestimation is nearly independent of the kinetic energy. The absolute value of the average electronic coupling matrix element increases with the reduced mass while that of the vibrational overlap integral decreases with the reduced mass. This accidental substantial cancellation in the − system is responsible for the experimental observation that the relative predissociation rate of two isotopologues approximately equals the squared inverse reduced mass ratio. The origin and implications of the isotope dependence of the averaged electronic coupling matrix elements are discussed.

Adaptive decision feedback equalizer with dynamic principle for echo cancellation

Adaptive decision feedback equalizer (ADFE) based on Least Mean square algorithm (LMS) has emerged as a novel tool for signal propagation without noise. However, it has the limitation of computational complexity. Since it has been reported that substantial echo cancellation leads to increased system quality and capacity, this paper proposes a dynamic LMS-based ADFE (DLMS-ADFE) to cancel the generation of the echo signal. The DLMS-ADFE has two dynamic principles in updating the coefficients. The first principle has varying step-size and the second principle has varying degree and magnitude of the signal. The comparison results show that the DLMS-ADFE is outperforming LMS-ADFE.

Evaluating rainfall errors in global climate models through cloud regimes

Global climate models suffer from a persistent shortcoming in their simulation of rainfall by producing too much drizzle and too little intense rain. This erroneous distribution of rainfall is a result of deficiencies in the representation of underlying processes of rainfall formation. In the real world, clouds are precursors to rainfall and the distribution of clouds is intimately linked to the rainfall over the area. This study examines the model representation of tropical rainfall using the cloud regime concept. In observations, these cloud regimes are derived from cluster analysis of joint-histograms of cloud properties retrieved from passive satellite measurements. With the implementation of satellite simulators, comparable cloud regimes can be defined in models. This enables us to contrast the rainfall distributions of cloud regimes in 11 CMIP5 models to observations and decompose the rainfall errors by cloud regimes. Many models underestimate the rainfall from the organized convective cloud regime, which in observation provides half of the total rain in the tropics. Furthermore, these rainfall errors are relatively independent of the model’s accuracy in representing this cloud regime. Error decomposition reveals that the biases are compensated in some models by a more frequent occurrence of the cloud regime and most models exhibit substantial cancellation of rainfall errors from different regimes and regions. Therefore, underlying relatively accurate total rainfall in models are significant cancellation of rainfall errors from different cloud types and regions. The fact that a good representation of clouds does not lead to appreciable improvement in rainfall suggests a certain disconnect in the cloud-precipitation processes of global climate models.

Newtonian-noise cancellation in large-scale interferometric GW detectors using seismic tiltmeters

The mitigation of terrestrial gravity noise, also known as Newtonian noise (NN), is one of the foremost challenges to improve low-frequency sensitivity of ground-based gravitational-wave detectors. At frequencies above 1 Hz, it is predicted that gravity noise from seismic surface Rayleigh waves is the dominant contribution to NN in surface detectors, and may still contribute significantly in future underground detectors. Noise cancellation based on a coherent estimate of NN using data from a seismometer array was proposed in the past. In this article, we propose an alternative scheme to cancel NN using a seismic tiltmeter. It is shown that even under pessimistic assumptions concerning the complexity of the seismic field, a single tiltmeter under each test mass of the detector is sufficient to achieve substantial noise cancellation. A technical tiltmeter design is presented to achieve the required sensitivity in the Newtonian-noise frequency band.

Analysis of immobilized artificial membrane retention factors for both neutral and ionic species

Retention data on an immobilised artificial membrane have been taken from the work of Li et al. and from Liu et al., and have been correlated with a set of descriptors that includes descriptors for ionized species, that is anions from deprotonated acids and cations from protonated bases. Logk(IAM) values can be predicted for acids or bases that are partially ionized at the experimental pH and logk(IAM) values for acids and bases can be predicted as a function of the fraction present as the ionized species, equivalent to prediction as a function of pH. It is shown that anions reduce the value of logk(IAM) by about 1.1log units but that cations have almost no effect by comparison to the neutral species. By comparison to non-polar solvents, carboxylate anions and protonated base cations are considerably stabilized by both water and the IAM phase to about 6–8log units and so the rather small influence of anions (−1.1log unit) and cations (−0.1log unit) on logk(IAM) is due to substantial cancellation of these stabilization effects. Indeed, the effect of change of phase from water to IAM on the neutral species is at least as large as the effect of change of phase on the ionic species.

Multiplicative mimicry and improvements to the Pólya–Vinogradov inequality

We study exponential sums whose coefficients are completely multiplicative and belong to the complex unit disc. Our main result shows that such a sum has substantial cancellation unless the coefficient function is essentially a Dirichlet character. As an application we improve current bounds on odd-order character sums. Furthermore, conditionally on the generalized Riemann hypothesis we obtain a bound for odd-order character sums which is best possible.

Free energy simulations of a GTPase: GTP and GDP binding to archaeal initiation factor 2.

Archaeal initiation factor 2 (aIF2) is a protein involved in the initiation of protein biosynthesis. In its GTP-bound, “ON” conformation, aIF2 binds an initiator tRNA and carries it to the ribosome. In its GDP-bound, “OFF” conformation, it dissociates from tRNA. To understand the specific binding of GTP and GDP and its dependence on the ON or OFF conformational state of aIF2, molecular dynamics free energy simulations (MDFE) are a tool of choice. However, the validity of the computed free energies depends on the simulation model, including the force field and the boundary conditions, and on the extent of conformational sampling in the simulations. aIF2 and other GTPases present specific difficulties; in particular, the nucleotide ligand coordinates a divalent Mg2+ ion, which can polarize the electronic distribution of its environment. Thus, a force field with an explicit treatment of electronic polarizability could be necessary, rather than a simpler, fixed charge force field. Here, we begin by comparing a fixed charge force field to quantum chemical calculations and experiment for Mg2+:phosphate binding in solution, with the force field giving large errors. Next, we consider GTP and GDP bound to aIF2 and we compare two fixed charge force fields to the recent, polarizable, AMOEBA force field, extended here in a simple, approximate manner to include GTP. We focus on a quantity that approximates the free energy to change GTP into GDP. Despite the errors seen for Mg2+:phosphate binding in solution, we observe a substantial cancellation of errors when we compare the free energy change in the protein to that in solution, or when we compare the protein ON and OFF states. Finally, we have used the fixed charge force field to perform MDFE simulations and alchemically transform GTP into GDP in the protein and in solution. With a total of about 200 ns of molecular dynamics, we obtain good convergence and a reasonable statistical uncertainty, comparable to the force field uncertainty, and somewhat lower than the predicted GTP/GDP binding free energy differences. The sign and magnitudes of the differences can thus be interpreted at a semiquantitative level, and are found to be consistent with the experimental binding preferences of ON- and OFF-aIF2.

Action at Hooked or Twisted–Hooked DNA Juxtapositions Rationalizes Unlinking Preference of Type-2 Topoisomerases

The mathematical basis of the hypothesis that type-2 topoisomerases recognize and act at specific DNA juxtapositions has been investigated by coarse-grained lattice polymer models, showing that selective segment passages at hooked juxtapositions can result in dramatic reductions in catenane and knot populations. The lattice modeling approach is here extended to account for the narrowing of variance of linking number ( Lk) of DNA circles by type-2 topoisomerases. In general, the steady-state variance of Lk resulting from selective segment passages at a specific juxtaposition geometry j is inversely proportional to the average linking number, 〈 Lk〉 j , of circles with the given juxtaposition. Based on this formulation, we demonstrate that selective segment passages at hooked juxtapositions reduce the variance of Lk. The dependence of this effect on model DNA circle size is remarkably similar to that observed experimentally for type-2 topoisomerases, which appear to be less capable in narrowing Lk variance for small DNA circles than for larger DNA circles. This behavior is rationalized by a substantial cancellation of writhe in small circles with hook-like juxtapositions. During our simulations, we uncovered a twisted variation of the hooked juxtaposition that has an even more dramatic effect on Lk variance narrowing than the hooked juxtaposition. For an extended set of juxtapositions, we detected a significant correlation between the Lk narrowing potential and the logarithmic decatenating and unknotting potentials for a given juxtaposition, a trend reminiscent of scaling relations observed with experimental measurements on type-2 topoisomerases from a variety of organisms. The consistent agreement between theory and experiment argues for type-2 topoisomerase action at hooked or twisted–hooked DNA juxtapositions.

Cancellation of EEG and MEG signals generated by extended and distributed sources.

Extracranial patterns of scalp potentials and magnetic fields, as measured with electro- and magnetoencephalography (EEG, MEG), are spatially widespread even when the underlying source in the brain is focal. Therefore, loss in signal magnitude due to cancellation is expected when multiple brain regions are simultaneously active. We characterized these cancellation effects in EEG and MEG using a forward model with sources constrained on an anatomically accurate reconstruction of the cortical surface. Prominent cancellation was found for both EEG and MEG in the case of multiple randomly distributed source dipoles, even when the number of simultaneous dipoles was small. Substantial cancellation occurred also for locally extended patches of simulated activity, when the patches extended to opposite walls of sulci and gyri. For large patches, a difference between EEG and MEG cancellation was seen, presumably due to selective cancellation of tangentially vs. radially oriented sources. Cancellation effects can be of importance when electrophysiological data are related to hemodynamic measures. Furthermore, the selective cancellation may be used to explain some observed differences between EEG and MEG in terms of focal vs. widespread cortical activity.

Partial cancellation of correlations in the nonrelativistic high-energy asymptotics of the photoeffect

We investigate the total effect of correlations on photoionization of atomic states with nonzero orbital momentum in the nonrelativistic high-energy asymptotic limit, considering the exclusive case of the dominant final state of an initial neutral atom. We find that the substantial cancellation of the dominant intrashell correlations, which had been reported earlier, can be understood utilizing the closure properties satisfied by the eigenfunctions of the nonrelativistic Hamiltonian. Considering the sum of correlations with all states, occupied or not, we show that complete sum is equal to the contribution of the high-energy part of the continuum. Consequently there is a total cancellation between the contributions of the bound states (occupied and unoccupied) and the low-energy part of the continuum states. This means that the real correlations in the physical atom due to the sum rule over the occupied states can be also obtained as the negative of the sum of contributions of low-energy bound and continuum unoccupied states. We calculate this in the framework of the quantum-defect model. As we would expect, the results are close to those obtained earlier in particular cases by direct summation over the occupied states. However this approach also allows us to see that the summore » of correlations over the occupied states is smaller than the dominant intrashell correlation. Hence, there is a partial cancellation between correlations with the occupied states. We can also obtain some limits on the correlation effects by considering calculations with the screened Coulomb functions. The role of correlations in the exclusive photoionization processes, also the modification of correlations in the case of atomic ions, is discussed.« less

An active vibration control model for coupled flexible systems

This paper presents a novel general model for complex flexible coupled systems. In this model, parallel structures of force actuators and passive spring isolators are installed between the machine and the foundation, and some moment actuators such as piezoelectric patches are installed on the flexible foundation whose vibration cancellation feature is the key object of vibration control. This model combines active and passive control, force and moment control into a single unit to achieve the efficient vibration control of flexible structures by multiple approaches. The state-space governing equations of the coupled system are deduced. Based on the description of the state-space equation of the coupled system, the transmission paths for the power flow transmitted into the foundation are discussed in the frequency domain, and then combined into a single function. The function includes two parts: the passive and active terms, which can be conveniently employed in an optimal control strategy to achieve power flow control. The transmission characteristics of the power flow by optimal control are discussed in detail. Numerical simulations are presented to show that both force and moment controls in the analytical model can achieve substantial vibration cancellation.

Helical Polyacetylenes Synthesized in Helical Sense and Pitch Controllable Chiral Nematic Liquid Crystal with Unprecedented Temperature Dependence

We have synthesized two types of novel chiral compounds, D-1 and D-2−D-5, where D-1 is an asymmetric center-containing chiral compound of (S)-configuration, and D-2−D-5 are so-called double chiral compounds consisting of axially chiral binaphthyl moiety of (S)-configuration and asymmetric center moieties of (S)-, (S)-, (R)- and racemic configuration, linked with methylene spacers of 6, 12, 12 and 12, respectively. These chiral compounds were adopted as chiral dopants to induce chiral nematic liquid crystals (N*-LCs). The N*-LC including the double chiral dopant D-3, abbreviated as N*-LC with D-3 (system 2), showed an extraordinary increase in helical pitch (decrease in helical twisting power) in polarizing optical microscope (POM) when the temperature increased from 0 to 30 °C. This is due to a substantial cancellation in twisting power between the double chiral moieties having opposite screw directions, although both of them have the same (S)-configuration. Meanwhile, the N*-LC including the two types of chiral dopants D-1 and D-3, abbreviated as N*-LC with both D-1 and D-3 (system 3), exhibited a temperature-dependent helical inversion; It showed a notable increase in left-handed helical pitch when the temperature increased from 0 to 15 °C, and became a nematic phase giving no helicity in the temperature range of 16−19 °C, and then showed a right-handed N* phase giving a decrease in helical pitch when the temperature increased from 20 to 25 °C. The changes in helical sense and helical pitch in the N*-LC were found to be thermally reversible. The helical pitch-controllable N*-LC (system 2) enabled us to control the screw structure and the spiral morphology of the helical polyacetylene by changing only the polymerization temperature.