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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 10252-10266 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Statistical ensembles for simulating liquid interfaces at constant pressure and/or surface tension are examined, and equations of motion for molecular dynamics are obtained by various extensions of the Andersen extended system approach. Valid ensembles include: constant normal pressure and surface area; constant tangential pressure and length normal to the interface; constant volume and surface tension; and constant normal pressure and surface tension. Simulations at 293 K and 1 atm normal pressure show consistent results with each other and with a simulation carried out at constant volume and energy. Calculated surface tensions for octane/water (61.5 dyn/cm), octane/vacuum (20.4 dyn/cm) and water/vacuum (70.2 dyn/cm) are in very good agreement with experiment (51.6, 21.7, and 72.8 dyn/cm, respectively). The practical consequences of simulating with two other approaches commonly used for isotropic systems are demonstrated on octane/water: applying equal normal and tangential pressures leads to an instability; and applying a constant isotropic pressure of 1 atm leads to a large positive normal pressure. Both results are expected for a system of nonzero surface tension. Mass density and water polarization profiles in the liquid/liquid and liquid/vapor interfaces are also compared.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 4613-4621 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for performing molecular dynamics simulations under constant pressure is presented. In the method, which is based on the extended system formalism introduced by Andersen, the deterministic equations of motion for the piston degree of freedom are replaced by a Langevin equation; a suitable choice of collision frequency then eliminates the unphysical "ringing'' of the volume associated with the piston mass. In this way it is similar to the "weak coupling algorithm'' developed by Berendsen and co-workers to perform molecular dynamics simulation without piston mass effects. It is shown, however, that the weak coupling algorithm induces artifacts into the simulation which can be quite severe for inhomogeneous systems such as aqueous biopolymers or liquid/liquid interfaces.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 2198-2202 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 117 (1995), S. 5561-5572 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 7070-7084 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A uniform neutralizing background plasma is employed by default in standard Ewald calculations for net-charged systems. We show here that this plasma leads to serious artifacts in both system energy and pressure, which lead to unrealistic behavior. These artifacts are especially critical to simulations where either net charge or volume is allowed to change. To correct these problems we institute a net-charge correction term that consists of subtracting off the Ewald sum for a single particle with charge equal to the net charge of the full system and an optional Born or Poisson–Boltzmann term. This correction decreases pressure artifacts by three orders of magnitude and allows rapid energy convergence as the cell size increases. The correction term is general, in that it applies to systems of any shape or net charge. The net-charge correction was tested for systems in all three phases of matter: gas, solid, and liquid, and found to be markedly superior to standard Ewald in all three cases. In the gas phase, isolated molecular energies are quickly achieved as the cell size increases. In the solid phase the vaporization energy of the NaCl crystal is reproduced using free energy perturbation techniques where a single atom is removed. In the liquid phase, the solvation free energy of Na+ was investigated. It is demonstrated that both the net-charge correction and an energy term twice the size of a Born term is required to obtain optimal free energies of ionic hydration. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 101 (1979), S. 1638-1639 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 101 (1979), S. 307-311 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 99 (1998), S. 279-288 
    ISSN: 1432-2234
    Keywords: Key words: Molecular dynamics ; Biomolecular simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. Coupled advances in empirical force fields and classical molecular dynamics simulation methodologies, combined with the availability of faster computers, has lead to significant progress towards accurately representing the structure and dynamics of biomolecular systems, such as proteins, nucleic acids, and lipids in their native environments. Thanks to these advances, simulation results are moving beyond merely evaluating force fields, displaying expected structural fluctuations, or demonstrating low root-mean-squared deviations from experimental structures and now provide believable structural insight into a variety of processes such as the stabilization of A-DNA in mixed water and ethanol solution or reversible β-peptide folding in methanol. The purpose of this overview is to take stock of these recent advances in biomolecular simulation and point out some common deficiencies exposed in longer simulations. The most significant methodological advances relate to the development of fast methods to properly treat long-range electrostatic interactions, and in this regard the fast Ewald methods are becoming the de facto standard.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular dynamics using CHARMM and GEMM programs with the Star Technologies ST 100 array processor functioning at the speed of super computers was used as a searching algorithm for conformational exploration of the octapeptide Gly-Asn-Thr-Ile-Val-Ala-Glu. This poorly soluble octapeptide is the N-terminal epitope of an 11 KD glycoprotein antigen residing on human ductal carcinoma (breast) cells. Very long (nanoseconds) simulations were required. Both an α-helix and the N-acetyl-N1-methylamide derived minimized starting structures gave the same lowest potential energy conformation with simulations at 600 K. The same conformation was found only when using the latter starting conformation with simulations at 300 K. The lowest potential energy conformation was stabilized by 4 hydrophobic contacts and 13 H bonds completing one turn of a left-handed helix.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0006-3525
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rate constant for the transition between the equatorial and axial conformations of N-acetylalanyl-N′-methylamide has been determined from Langevin dynamics (LD) simulations with no explicit solvent. The isomerization rate is maximum at collision frequency γ = 2 ps-1, shows diffusive character for γ ≥ 10 ps-1, but does not approach zero even at γ = 0.01 ps-1. This behavior differs from that found for a one-dimensional bistable potential and indicates that both collisional energy transfer with solvent and vibrational energy transfer between internal modes are important in the dynamics of barrier crossing for this system. It is suggested that conformational searches of peptides be carried out using LD with a collision frequency that maximizes the isomerization rate (i.e., γ ≈ 2 ps-1). This method is expected to be more efficient than either molecular dynamics in vacuo (which corresponds to LD with γ = 0) or molecular dynamics in solvent (where dynamics is largely diffusive).
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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