Pressure Calculation in Polar and Charged Systems using Ewald Summation: Results for the Extended Simple Point Charge Model of Water

Ewald summation and physically equivalent methods such as particle-mesh Ewald, kubic-harmonic expansions, or Lekner sums are commonly used to calculate long-range electrostatic interactions in computer simulations of polar and charged substances. The calculation of pressures in such systems is investigated. We find that the virial and thermodynamic pressures differ because of the explicit volume dependence of the effective, resummed Ewald potential. The thermodynamic pressure, obtained from the volume derivative of the Helmholtz free energy, can be expressed easily for both ionic and rigid molecular systems. For a system of rigid molecules, the electrostatic energy and the forces at the atom positions are required, both of which are readily available in molecular dynamics codes. We then calculate the virial and thermodynamic pressures for the extended simple point charge (SPC/E) water model at standard conditions. We find that the thermodynamic pressure exhibits considerably less system size dependence than the virial pressure. From an analysis of the cross correlation between the virial and thermodynamic pressure, we conclude that the thermodynamic pressure should be used to drive volume fluctuations in constant-pressure simulations.Comment: RevTeX, 19 pages, 2 EPS figures; in press: Journal of Chemical Physics, 15-August-199

The Ursinus Weekly, October 6, 1905

Football • Alumni • Society notes • Old fashioned corn roast • Oyster supper • College world • School of Theology • Songshttps://digitalcommons.ursinus.edu/weekly/2951/thumbnail.jp

The Ursinus Weekly, June 9, 1905

Baccalaureate sermon • Class Day exercises • Commencement • Reunion • Junior oratorical contest • Society notes • College notes • Farewell reception • Baseball • Alumni Day • Alumni oration • President\u27s receptionhttps://digitalcommons.ursinus.edu/weekly/3019/thumbnail.jp

The Ursinus Weekly, March 2, 1906

1907 stag party • Intercollegiate oratorical contest • Mr. Keiner entertains • Alumni • Baseball outlook • Society notes • Glee and orchestra at Conshohocken • Philadelphia letter • Personalshttps://digitalcommons.ursinus.edu/weekly/2971/thumbnail.jp

The Ursinus Weekly, April 21, 1905

Baseball • The Reverend J. H. Sechler, D. D. • Resolutions • Society notes • College sermon • Seminary notes • Alumni noteshttps://digitalcommons.ursinus.edu/weekly/3012/thumbnail.jp

Resonant Clumping and Substructure in Galactic Discs

We describe a method to extract resonant orbits from N-body simulations exploiting the fact that they close in a frame rotating with a constant pattern speed. Our method is applied to the N-body simulation of the Milky Way by Shen et al. (2010). This simulation hosts a massive bar, which drives strong resonances and persistent angular momentum exchange. Resonant orbits are found throughout the disc, both close to the bar itself and out to the very edges of the disc. Using Fourier spectrograms, we demonstrate that the bar is driving kinematic substructure even in the very outer parts of the disc. We identify two major orbit families in the outskirts of the disc that make significant contributions to the kinematic landscape, namely the m:l = 3:-2 and 1:-1 families resonating with the pattern speed of the bar. A mechanism is described that produces bimodal distributions of Galactocentric radial velocities at selected azimuths in the outer disc. It occurs as a result of the temporal coherence of particles on the 3:-2 resonant orbits, which causes them to arrive simultaneously at pericentre or apocentre. This resonant clumping, due to the in-phase motion of the particles through their epicycle, leads to both inward and outward moving groups which belong to the same orbital family and consequently produce bimodal radial velocity distributions. This is a possible explanation of the bimodal velocity distributions observed towards the Galactic anti-Centre by Liu et al. (2012). Another consequence is that transient overdensities appear and dissipate (in a symmetric fashion) on timescales equal to the their epicyclic period resulting in a periodic pulsing of the disc's surface density.Comment: 11 Figures, 1 Table. Accepted for publication in ApJ. Version 2 reflects minor changes to the text. Animation referenced in Figure 7 is available at http://hubble.shao.ac.cn/~shen/resonantclumping/DensMovie.mp

The Ursinus Weekly, February 9, 1906

The Angelus by Millet • Characters of The Bells entertained • Administration Committee • Society notes • Philadelphia letter • A glance in passing • Union programhttps://digitalcommons.ursinus.edu/weekly/2968/thumbnail.jp

The Ursinus Weekly, November 10, 1905

Football • Alumni • Week of Prayer • Seniors • Ursinus Union • Society notes • College notes • College world • Gettysburg game • New football songhttps://digitalcommons.ursinus.edu/weekly/2956/thumbnail.jp

The Ursinus Weekly, May 26, 1905

Baseball • Junior boating party • Alumni notes • Commencement • Pupils\u27 recital • Society notes • College notes • Noticehttps://digitalcommons.ursinus.edu/weekly/3017/thumbnail.jp