Noise-induced vortex reversal of self-propelled particles

We report an interesting phenomenon of noise-induced vortex reversal in a two-dimensional system of self-propelled particles (SPP) with soft-core interactions. With the aid of forward flux sampling, we analyze the configurations along the reversal pathway and thus identify the mechanism of vortex reversal. We find that statistically the reversal exhibits a hierarchical process: those particles at the periphery first change their motion directions, and then more inner layers of particles reverse later on. Furthermore, we calculate the dependence of the average reversal rate on noise intensity $D$ and the number $N$ of SPP. We find that the rate decreases exponentially with the reciprocal of $D$. Interestingly, the rate varies nonmonotonically with $N$ and a minimal rate exists for an intermediate value of $N$.Comment: 4 pages, 5 figure

Green-Function-Based Monte Carlo Method for Classical Fields Coupled to Fermions

Microscopic models of classical degrees of freedom coupled to non-interacting fermions occur in many different contexts. Prominent examples from solid state physics are descriptions of colossal magnetoresistance manganites and diluted magnetic semiconductors, or auxiliary field methods for correlated electron systems. Monte Carlo simulations are vital for an understanding of such systems, but notorious for requiring the solution of the fermion problem with each change in the classical field configuration. We present an efficient, truncation-free O(N) method on the basis of Chebyshev expanded local Green functions, which allows us to simulate systems of unprecedented size N.Comment: 4 pages, 3 figure

Approximation of the scattering amplitude

The simultaneous solution of Ax=b and ATy=g is required in a number of situations. Darmofal and Lu have proposed a method based on the Quasi-Minimal residual algorithm (QMR). We will introduce a technique for the same purpose based on the LSQR method and show how its performance can be improved when using the Generalized LSQR method. We further show how preconditioners can be introduced to enhance the speed of convergence and discuss different preconditioners that can be used. The scattering amplitude gTx, a widely used quantity in signal processing for example, has a close connection to the above problem since x represents the solution of the forward problem and g is the right hand side of the adjoint system. We show how this quantity can be efficiently approximated using Gauss quadrature and introduce a Block-Lanczos process that approximates the scattering amplitude and which can also be used with preconditioners

The Child is Father of the Man: Foresee the Success at the Early Stage

Understanding the dynamic mechanisms that drive the high-impact scientific work (e.g., research papers, patents) is a long-debated research topic and has many important implications, ranging from personal career development and recruitment search, to the jurisdiction of research resources. Recent advances in characterizing and modeling scientific success have made it possible to forecast the long-term impact of scientific work, where data mining techniques, supervised learning in particular, play an essential role. Despite much progress, several key algorithmic challenges in relation to predicting long-term scientific impact have largely remained open. In this paper, we propose a joint predictive model to forecast the long-term scientific impact at the early stage, which simultaneously addresses a number of these open challenges, including the scholarly feature design, the non-linearity, the domain-heterogeneity and dynamics. In particular, we formulate it as a regularized optimization problem and propose effective and scalable algorithms to solve it. We perform extensive empirical evaluations on large, real scholarly data sets to validate the effectiveness and the efficiency of our method.Comment: Correct some typos in our KDD pape

The Numerical Estimation of the Error Induced by the Valence Approximation

We describe a systematic expansion for full QCD. The leading term in the expansion gives the valence approximation. The expansion reproduces full QCD if an infinite number of higher terms are included.Comment: 3 pages, latex, no figures, requires espcrc2.sty (included at end) Contribution to Lattice 94 proceeding

Strange But True: Claim Social Security Now, Claim More Later

Under Social Security, married individuals are entitled to a retired worker benefit based on their own earnings and/or to a spousal benefit equal to one half of their spouse’s benefit claimed at the Full Retirement Age (currently 66). If a married individual claims before the Full Retirement Age, the Social Security Administration assumes that the individual is claiming both types of benefits, compares the worker and spousal benefits, and awards the highest. Upon reaching the Full Retirement Age, individuals can choose which benefit to receive. As a result, married individuals can claim a spousal benefit at 66 and switch to their own retired worker benefit at a later date. This approach allows a worker to begin claiming one type of benefit while still building up delayed retirement credits, which will result in a higher worker benefit later. In the past, providing these benefit options for spouses was not particularly valuable, since those who postponed benefits beyond the Full Retirement Age were giving up expected lifetime benefits. With the recent advent of an actuarially fair delayed retirement credit, lifetime benefits are roughly the same whether claimed at the Full Retirement Age or at age 70. As a result, today the availability of benefit options has real value for couples and therefore inevitably increases the cost of the Social Security program...

Strange But True: Claim and Suspend Social Security

With the current financial crisis wreaking havoc on retirement savings, many older people have had to reassess their retirement plans – they may decide to work longer or, if already retired, to re-enter the workforce. For those currently in the labor force, working longer increases monthly Social Security benefits. Social Security benefits are actuarially adjusted so that, on average, lifetime benefits remain the same whether a person retires at any age between 62 and 70. So the later a person retires, the higher the monthly benefit. For those thinking of re-entering the workforce, Social Security provides for higher benefits later in exchange for withholding benefits while they are employed. For those under the Full Retirement Age (currently 66), this adjustment is accomplished automatically through the annual retirement earnings test. For those over the Full Retirement Age, the adjustment can be made through the voluntary option of “claim and suspend.” The “claim and suspend” strategy also enhances the claiming options of one-earner couples. For example, a husband who reaches the Full Retirement Age may elect to claim and immediately suspend benefits, allowing his wife to receive a spousal benefit based on his earnings record. The husband is then free to continue working and receive delayed retirement credits, which increases not only his monthly benefit but also his wife’s survivor benefit. By using “claim and suspend” in this way, the couple can enhance the value of their lifetime benefits...

Resolving the notorious case of conical intersections for coupled cluster dynamics

The motion of electrons and nuclei in photochemical events often involve conical intersections, degeneracies between electronic states. They serve as funnels for nuclear relaxation - on the femtosecond scale - in processes where the electrons and nuclei couple nonadiabatically. Accurate ab initio quantum chemical models are essential for interpreting experimental measurements of such phenomena. In this paper we resolve a long-standing problem in coupled cluster theory, presenting the first formulation of the theory that correctly describes conical intersections between excited electronic states of the same symmetry. This new development demonstrates that the highly accurate coupled cluster theory can be applied to describe dynamics on excited electronic states involving conical intersections.Comment: 8 pages and 3 figures and including supporting information (with corrections and improved notation

How Much Risk is Acceptable?

The financial crisis has sparked proposals to reform the retirement income system. One component of such a system could be a new tier of retirement accounts. These accounts would augment declining Social Security replacement rates for low-wage workers and provide a buffer of security for middle- and upper-wage workers who, increasingly, will rely totally on 401(k) plans to supplement their Social Security. Designing such a new tier requires answering a number of questions: Mandatory or voluntary? Employee and/or employer contributions? Subsidies for low earners? Payments as lump sums or annuities? Tax favored or not? But the most fundamental question is whether the goal of the new tier is to provide a defined contribution account, where the retirement income will depend on market performance, or an account that can provide a certain percent of final earnings ñ that is, a target replacement rate...

Chiral properties of two-flavor QCD in small volume and at large lattice spacing

We present results from simulations of two flavors of dynamical overlap fermions on 8^4 lattices at three values of the sea quark mass and a lattice spacing of about 0.16 fm. We measure the topological susceptibility and the chiral condensate. A comparison of the low-lying spectrum of the overlap operator with predictions from random matrix theory is made. To demonstrate the effect of the dynamical fermions, we compare meson two-point functions with quenched results. Algorithmic improvements over a previous publication and the performance of the algorithm are discussed.Comment: 16 pages, 12 figure