Student-Led Webinar to Support LGBTQ+ Students Applying to Medical School During the COVID-19 Pandemic

Lesbian, gay, bisexual, transgender, queer, intersex, asexual, non-binary, two-spirit, and other (LGBTQ+) students are a diverse group with unique and frequently overlooked needs in medical training. The present study was designed to understand the concerns of LGBTQ+ applicants to medical school and examine the effectiveness of a webinar in alleviating concerns. Sixty participants joined webinars discussing the medical school application process with particular attention to concerns pertinent to the LGBTQ+ population. Pre and post surveys were administered to examine webinar effectiveness and participant concerns. Results were analyzed using quantitative and qualitative methods. Pre-medical students reported that the webinar format was helpful for their application process. Specifically, pre- and post-test analyses revealed that the webinar increased both students' preparedness as well as their confidence in disclosing their LGBTQ+ identity or being "out" when applying to medical school. Student-led, online webinars increase LGBTQ+ students' confidence and help address SGM students' concerns about applying to medical school.

Low-cost PM2.5 sensors can help identify driving factors of poor air quality and benefit communities

Air quality is critical for public health. Residents rely chiefly on government agencies such as the Environmental Protection Agency (EPA) in the United States to establish standards for the measurement of harmful contaminants including ozone, sulfur dioxide, carbon monoxide, volatile organic chemicals (VOCs), and fine particulate matter at or below 2.5 μm. According to the California Air Resources Board [1], "short-term PM2.5 exposure (up to 24-h duration) has been associated with premature mortality, increased hospital admissions for heart or lung causes, acute and chronic bronchitis, asthma attacks, emergency room visits, respiratory symptoms, and restricted activity days". While public agency resources may provide guidance, it is often inadequate relative to the widespread need for effective local measurement and management of air quality risks. To that end, this paper explores the use of low-cost PM2.5 sensors for measuring air quality through micro-scale (local) analytical comparisons with reference grade monitors and identification of potential causal factors of elevated sensor readings. We find that a) there is high correlation between the PM2.5 measurements of low-cost sensors and reference grade monitors, assessed through calibration models, b) low-cost sensors are more prevalent and provide more frequent measurements, and c) low-cost sensor data enables exploratory and explanatory analytics to identify potential causes of elevated PM2.5 readings. This understanding should encourage community scientists to place more low-cost sensors in their neighborhoods, which can empower communities to demand policy changes that are necessary to reduce particle pollution, and provide a basis for subsequent research.

The annual cycle for whimbrel populations using the Western Atlantic Flyway

Many long-distance migratory birds use habitats that are scattered across continents and confront hazards throughout the annual cycle that may be population-limiting. Identifying where and when populations spend their time is fundamental to effective management. We tracked 34 adult whimbrels (Numenius phaeopus) from two breeding populations (Mackenzie Delta and Hudson Bay) with satellite transmitters to document the structure of their annual cycles. The two populations differed in their use of migratory pathways and their seasonal schedules. Mackenzie Delta whimbrels made long (22,800 km) loop migrations with different autumn and spring routes. Hudson Bay whimbrels made shorter (17,500 km) and more direct migrations along the same route during autumn and spring. The two populations overlap on the winter grounds and within one spring staging area. Mackenzie Delta whimbrels left the breeding ground, arrived on winter grounds, left winter grounds and arrived on spring staging areas earlier compared to whimbrels from Hudson Bay. For both populations, migration speed was significantly higher during spring compared to autumn migration. Faster migration was achieved by having fewer and shorter stopovers en route. We identified five migratory staging areas including four that were used during autumn and two that were used during spring. Whimbrels tracked for multiple years had high (98%) fidelity to staging areas. We documented dozens of locations where birds stopped for short periods along nearly all migration routes. The consistent use of very few staging areas suggests that these areas are integral to the annual cycle of both populations and have high conservation value.

An enhanced procedure for urban mobile methane leak detection

Leaked methane from natural gas distribution pipelines is a significant human and environmental health problem in urban areas. To assess this risk, urban mobile methane leak surveys were conducted, using innovative methodology, on the streets of Hartford, Danbury, and New London, Connecticut, in March 2019. The Hartford survey was done to determine if results from a 2016 survey (Keyes et al., 2019) were persistent, and surveys in additional towns were done to determine if similar findings could be made using an identical approach. Results show that Hartford continues to be problematic, with approximately 3.4 leaks per road mile observed in 2016 and 4.3 leaks per mile estimated in 2019, similar to that previously found in Boston, Massachusetts (Phillips et al., 2013). A preliminary estimate of methane leaks in Hartford is 0.86 metric tonnes per day (or 313 metric tonnes per year), equivalent to 42,840 cubic feet per day of natural gas, and a daily gas consumption of approximately 214 U.S. households. Moreover, the surveys and analyses done for Danbury and New London also reveal problematic leaks, particularly for Danbury with an estimated 3.6 leaks per mile. Although road miles covered in New London were more limited, the survey revealed leak-prone areas, albeit with a range of methane readings lower than those in Hartford and Danbury. Data collection methods for all studies is first reported here and are readily transferable to similar urban settings. This work demonstrates the actionable value that can be gained from data-driven evaluations of urban pipeline performance, and if supplemented with a map of leak-prone pipe geo-location, and information on pipeline operating pressures, will provide a spatial database facilitating proactive repair and replacement of leak-prone urban pipes, a considerable improvement compared to reactive mitigation of human-reported leaks. While this work pertains to the selected urban towns in the Northeast, it exemplifies issues and opportunities nationwide in the United States.

The proton momentum distribution in strongly H-bonded phases of water: a critical test of electrostatic models

Water is often viewed as a collection of monomers interacting electrostatically with each other. We compare the water proton momentum distributions from recent neutron scattering data with those calculated from two electronic structure-based models. We find that below 500 K these electrostatic models, one based on a multipole expansion, which includes the polarizability of the monomers, are not able to even qualitatively account for the sizable vibrational zero-point contribution to the enthalpy of vaporization. This discrepancy is evidence that the change in the proton well upon solvation cannot be entirely explained by electrostatic effects alone, but requires correlations of the electronic states on the molecules involved in the hydrogen bonds to produce the observed softening of the well.

Time correlation function and finite field approaches to the calculation of the fifth order Raman response in liquid xenon

The fifth order, two-dimensional Raman response in liquid xenon is calculated via a time correlation function (TCF) theory and the numerically exact finite field method. Both employ classical molecular dynamics simulations. The results are shown to be in excellent agreement, suggesting the efficacy of the TCF approach, in which the response function is written approximately in terms of a single classical multitime TCF.

On the mechanism of reorientational and structural relaxation in supercooled liquids: the role of border dynamics and cooperativity

Molecular dynamics simulation and analysis based upon the many-body potential energy landscape (PEL) are employed to characterize single molecule reorientation and structural relaxation, and their interrelation, in deeply supercooled liquid CS(2). The rotational mechanism changes from small-step Debye diffusion to sudden large angle reorientation (SLAR) as the temperature falls below the mode-coupling temperature T(c). The onset of SLAR is explained in terms of the PEL; it is an essential feature of low-T rotational dynamics, along with the related phenomena of dynamic heterogeneity and the bifurcation of slow and fast relaxation processes. A long trajectory in which the system is initially trapped in a low energy local minimum, and eventually escapes, is followed in detail, both on the PEL and in real space. During the trapped period, "return" dynamics occurs, always leading back to the trap. Structural relaxation is identified with irreversible escape to a new trap. These processes lead to weak and strong SLAR, respectively; strong SLAR is a clear signal of structural relaxation. Return dynamics involves small groups of two to four molecules, while a string-like structure composed of all the active groups participates in the escape. It is proposed that, rather than simple, nearly instantaneous, one-dimensional barrier crossings, relaxation involves activation of the system to the complex, multidimensional region on the borders of the basins of attraction of the minima for an extended period.

Theoretical Investigation of the Temperature Dependence of the Fifth-Order Raman Response Function of Fluid and Liquid Xenon

The temperature dependence of the fifth-order Raman response function, R(5)(t1,t2), is calculated for fluid xenon by employing a recently developed time-correlation function (TCF) theory. The TCF theory expresses the two-dimensional (2D) Raman quantum response function in terms of a two-time, computationally tractable, classical TCF. The theory was shown to be in excellent agreement with existing exact classical MD calculations for liquid xenon as well as reproducing line shape characteristics predicted by earlier theoretical work. It is applied here to investigate the temperature dependence of the fifth-order Raman response function in fluid xenon. In general, the characteristic line shapes are preserved over the temperature range investigated (for the reduced temperature points T* = 0.5, 1.0, and 2.0); differences in the signal decay times and a large decline in intensity with decreasing temperature (and associated anharmonicity) are observed. In addition, there are some signature features that were not observed in earlier results for T* = 1. The most dramatic difference in line shape is observed for the polarization condition, xxzzxx, that shows a vibrational echo peak. In contrast, the fully polarized signal changes mainly in magnitude.

Applications of a time correlation function theory for the fifth-order Raman response function I: Atomic liquids

Multidimensional spectroscopy has the ability to provide great insight into the complex dynamics and time-resolved structure of liquids. Theoretically describing these experiments requires calculating the nonlinear-response function, which is a combination of quantum-mechanical time correlation functions R5(t1,t2) was expressed with a two-time, computationally tractable, classical TCF. Writing the response function in terms of classical TCFs brings the full power of atomistically detailed molecular dynamics to the problem. In this paper, the new TCF theory is employed to calculate the fifth-order Raman response function for liquid xenon and investigate several of the polarization conditions for which experiments can be performed on an isotropic system. The theory is shown to reproduce line-shape characteristics predicted by earlier theoretical work.

On the Breakdown of the Stokes−Einstein Law in Supercooled Liquids

The wavevector-dependent shear viscosity, eta(k), is evaluated for a range of temperatures in a supercooled binary Lennard-Jones liquid. The mode coupling theory of Keyes and Oppenheim (Phys. Rev. A 1973, 8, 937) expresses the self-diffusion constant, D, in terms of eta(k). Replacing eta(k) with the usual viscosity, eta identical with eta(k = 0), yields the Stokes-Einstein law. It is found that the breakdown of the SE law in this system is well described by keeping the simulated k-dependence. Simply put, bath processes on all length scales (wavevectors) contribute to D, the system is much less viscous at finite k, and thus D exceeds the SE estimate based upon eta. The functional form of eta(k) allows for the estimation of a correlation length that grows with decreasing T.

Qualitative features of the two-dimensional Raman spectrum in liquids

The theory presented earlier [J. Kim and T. Keyes, Phys. Rev. E 66, 051110 (2002)] is analyzed to determine the information available from the two-dimensional Raman spectrum R((5))(t(2),t(1)) in liquids. The known spectra are well represented by the sum of two products of ordinary time correlations predicted by the theory. The shape of R((5)) is related in general to the values of simple same-time averages and concepts amenable to physical intuition. Using standard models for the time correlations entering the theory, specific analytic expressions for the spectrum are obtained depending on two parameters and a time scale, and the behavior of the spectrum is mapped out in the parameter space.

Tractable theory of nonlinear response and multidimensional nonlinear spectroscopy

Nonlinear spectroscopy provides insights into dynamics, but the response functions required for its interpretation pose a challenge to theorists. We proposed an approach in which the fifth-order response function [R5( t1, t2)] was expressed as a two-time classical time correlation function (TCF). Here, we present TCF theory results for R5( t1, t2) in liquid xenon. Using a first-order dipole-induced dipole polarizability model, the result is compared to an exact numerical calculation showing remarkable agreement. In addition, R5( t1, t2) is calculated using the exactly solved polarizability model, yielding different results and predicting an echo signal.

A time correlation function theory of two-dimensional infrared spectroscopy with applications to liquid water

A theory describing the third-order response function R((3))(t(1),t(2),t(3)), which is associated with two-dimensional infrared (2DIR) spectroscopy, has been developed. R((3)) can be written as sums and differences of four distinct quantum mechanical dipole (multi)time correlation functions (TCF's), each with the same classical limit; the combination of TCF's has a leading contribution of order variant Planck's over 2pi (3) and thus there is no obvious classical limit that can be written in terms of a TCF. In order to calculate the response function in a form amenable to classical mechanical simulation techniques, it is rewritten approximately in terms of a single classical TCF, B(R)(t(1),t(2),t(3))=micro(j)(t(2)+t(1))micro(i)(t(3)+t(2)+t(1))micro(k)(t(1))micro(l)(0), where the subscripts denote the Cartesian dipole directions. The response function is then given, in the frequency domain, as the Fourier transform of a classical TCF multiplied by frequency factors. This classical expression can then further be quantum corrected to approximate the true response function, although for low frequency spectroscopy no correction is needed. In the classical limit, R((3)) becomes the sum of multidimensional time derivatives of B(R)(t(1),t(2),t(3)). To construct the theory, the response function's four TCF's are rewritten in terms of a single TCF: first, two TCF's are eliminated from R((3)) using frequency domain detailed balance relationships, and next, two more are removed by relating the remaining TCF's to each other within a harmonic oscillator approximation; the theory invokes a harmonic approximation only in relating the TCF's and applications of theory involve fully anharmonic, atomistically detailed molecular dynamics (MD). Writing the response function as a single TCF thus yields a form amenable to calculation using classical MD methods along with a suitable spectroscopic model. To demonstrate the theory, the response function is obtained for liquid water with emphasis on the OH stretching portion of the spectrum. This approach to evaluating R((3)) can easily be applied to chemically interesting systems currently being explored experimentally by 2DIR and to help understand the information content of the emerging multidimensional spectroscopy.

Potential-energy-landscape-based extended van der Waals equation

The inherent structures (IS) are the local minima of the 3N-dimensional potential energy surface, or landscape, of an N-atom system. Stillinger has given an exact IS formulation of thermodynamics. Here the implications for the equation of state are investigated. It is shown that the van der Waals (vdW) equation, with density-dependent a and b coefficients, holds if the averaged IS energy is close to its high-temperature plateau value. The density-dependence alone significantly enriches the equation of state. Furthermore, an additional "landscape" contribution to the pressure is found at lower T. The resulting extended vdW equation is capable of yielding a waterlike density anomaly, flat isotherms in the coexistence region vs vdW loops, and several other desirable features. The plateau IS energy, the width of the distribution of IS, and T(TOL), the "top of the landscape" temperature at which the plateau is reached, are simulated over a broad reduced density range, 2.0>or=rho>or=0.20, in the Lennard-Jones fluid. Fits to the data yield an explicit equation of state, which is argued to be plausible at high density. Nevertheless, a(rho(c)) and b(rho(c)), where rho(c) is the critical density, are in excellent agreement with the standard values obtained by fitting the vdW equation at the critical point.

Probes of heterogeneity in rotational dynamics: application to supercooled liquid CS2

The distribution of individual molecular contributions to the second-rank rotational correlation function is introduced and used to construct probes of heterogeneity in rotational dynamics. The ideas are tested in a molecular dynamics simulation of supercooled liquid CS2. Both the quantity of heterogeneity and its lifetime or exchange time tau(ex) increase as the temperature is lowered through the supercooled state, and increase strongly as the mode-coupling temperature T(c) is approached. Crossover from Arrhenius to super-Arrhenius behavior of the rotational relaxation times tau(1) and tau(2) is observed, direct evidence of fragility in CS2. The T dependence of tau(ex) is stronger than that of the rotational times, and it may approach them from below at T(c), although the simulation is then very difficult. A detailed characterization of other aspects of the dynamical crossover is obtained, and the general implications of rotational heterogeneity for supercooled dynamics are discussed.

Random energy model for dynamics in supercooled liquids: N dependence

The random energy model (REM) for the critical points (saddles and minima) of the potential energy landscape of liquids is further developed. While thermodynamic properties may be calculated from the unconditional distribution of states G(E), dynamics requires the distribution G(c)(E';E) of energies E' of neighbors connected to a state with energy E. Previously it was shown [T. Keyes, Phys. Rev. E 62, 7905 (2000)] that an uncorrelated REM, G(c)(E';E)=G(E'), is badly behaved in the thermodynamic limit N--> infinity. In the following, a simple expression is obtained for G(c)(E';E), which leads to reasonable N dependences. Results are obtained for the fraction f(u) of imaginary-frequency instantaneous normal modes, the configuration entropy S(c), the distributions of the different-order critical points, and the rate R of escape from a state. Simulation data on f(u)(T) and the density of minima rho(0)(E) in Lennard-Jones and CS2 are fit with the theory, allowing a determination of some model parameters. A universal scaling form for f(u), and a consequent scheme for calculating the mode-coupling temperature T(c) consistently among different materials, is demonstrated. The dependence of the self-diffusion constant D upon R and f(u) is discussed, with the conclusion that D proportional, variant f(u) in deeply supercooled states. The phenomenology of fragile supercooled liquids is interpreted. It is shown that the REM need not have a Kauzmann transition in the relevant temperature range, i.e., above the glass transition.

Generalized Langevin equation approach to higher-order classical response: second-order-response time-resolved Raman experiment in CS2

A simple, systematic generalized Langevin equation approach for calculating classical nonlinear response functions is formulated and discussed. The two-time Poisson brackets appearing at second and higher order are rendered tractable by a physically motivated approximation. The method is used to calculate the fifth order (second order response) Raman response of liquid CS2. Agreement with simulation is good, but the simplicity of the theoretical expression suggests that the path to obtaining qualitatively new information about liquids with the fifth order experiment is uncertain. Further applications of the basic approach are suggested.

Potential energy landscape and mechanisms of diffusion in liquids

The mechanism of diffusion in supercooled liquids is investigated from the potential energy landscape point of view, with emphasis on the crossover from high- to low-T dynamics over the range T(A) > or =T > or =T(c). Molecular dynamics simulations with a time dependent mapping to the associated local minimum or inherent structure (IS) are performed on unit-density Lennard-Jones. Dynamical quantities introduced include r2(is)(t), the mean-square displacement (MSD) within a basin of attraction of an IS, R2(t), the MSD of the IS itself, and g(t), the distribution of IS waiting times. The configuration space is treated as a composite of the contributions of cooperative local regions, and a method is given to obtain the physically meaningful g(loc)(t) and mean waiting time tau(loc) from g(t). An understanding of the crossover is obtained in terms of r2(is)(t) and tau(loc). At intermediate T, r2(is)(t) possesses an interval of linear t dependence allowing calculation of an intrabasin diffusion constant D(is). Near T(c), where intrabasin diffusion is well established for t<tau(loc), diffusion is intrabasin dominated with D=D(is); D may be calculated within a basin. Below T(c), tau(loc) exceeds the time tau(pl) needed for the system to explore the basin, indicating the action of barriers at the border; tau(loc)=tau(pl) is a criterion for transition to activated hopping. Intrabasin diffusion provides a means of confinement not involving barriers and plays a key role in the dynamics above T(c). The distinction is discussed between motion among the IS (IS dynamics) below T(c) and saddle or border dynamics above T(c), where the system is always close to one of the saddle barriers connecting the basins and IS boundaries are closely spaced and easily crossed. A border index is introduced based upon the relation of R2(t) to the conventional MSD, and shown to vanish at T approximately T(c). It is proposed that intrabasin diffusion is a manifestation of saddle dynamics.

Conjugate gradient filtering of instantaneous normal modes, saddles on the energy landscape, and diffusion in liquids

Instantaneous normal modes (INM's) are calculated during a conjugate-gradient (CG) descent of the potential energy landscape, starting from an equilibrium configuration of a liquid or crystal. A small number (approximately equal to 4) of CG steps removes all the Im-omega modes in the crystal and leaves the liquid with diffusive Im-omega which accurately represent the self-diffusion constant D. Conjugate gradient filtering appears to be a promising method, applicable to any system, of obtaining diffusive modes and facilitating INM theory of D. The relation of the CG-step dependent INM quantities to the landscape and its saddles is discussed.

Inherent-structure dynamics and diffusion in liquids

The self-diffusion constant D is expressed in terms of transitions among the local minima (inherent structures, IS) of the N-body potential-energy surface or landscape, and their correlations. The formulas are evaluated and tested against simulation in the supercooled, unit-density Lennard-Jones liquid. The approximation of uncorrelated IS-transition (IST) vectors D0, greatly exceeds D for the highest T, but merges with simulation at reduced T approximately 0.50, close to the estimated mode-coupling temperature T(c). Since uncorrelated IST's are associated with a hopping mechanism, the condition D approximately D0 provides a new way to identify the crossover to hopping. The results suggest that theories of diffusion in deeply supercooled liquids may be based on weakly correlated IST's.

Entropy, dynamics, and instantaneous normal modes in a random energy model

It is shown that the fraction f(u) of imaginary-frequency instantaneous normal modes (INM) may be defined and calculated in a random energy model (REM) of liquids. The configurational entropy S(c) and the averaged hopping rate among the states, R, are also obtained and related to f(u) with the results R approximately f(u) and S(c)=a+b ln(f(u)). The proportionality between R and f(u) is the basis of existing INM theories of diffusion, so the REM further confirms their validity. A link to S(c) opens new avenues for introducing INM into dynamical theories. Liquid states are usually defined by assigning a configuration to the minimum to which it will drain, but the REM naturally treats saddle barriers on the same footing as minima, which may be a better mapping of the continuum of configurations to discrete states. Requirements for a detailed REM description of liquids are discussed.

Configurational entropy and collective modes in normal and supercooled liquids

Soft vibrational modes have been used to explain anomalous thermal properties of glasses above 1 K. The soft-potential model consists of a collection of double-well potentials that are distorted by a linear term representing local stress in the liquid. Double-well modes contribute to the configurational entropy of the system. Based on the Adam-Gibbs theory of entropically driven relaxation in liquids, we show that the presence of stress drives the transition from Arrhenius to Zwanzig-Bässler temperature dependence of relaxation times. At some temperature below the glass transition, the energy scale is dominated by local stress, and soft modes are described by single wells only. It follows that the configurational entropy vanishes, in agreement with the "Kauzmann paradox." We discuss a possible connection between soft vibrational modes and ultrafast processes that dominate liquid dynamics near the glass transition.