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  • 1
    Electronic Resource
    Electronic Resource
    The quasi-Gaussian entropy theory: Free energy calculations based on the potential energy distribution function (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 1560-1574 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new theory is presented for calculating the Helmholtz free energy based on the potential energy distribution function. The usual expressions of free energy, internal energy and entropy involving the partition function are rephrased in terms of the potential energy distribution function, which must be a near Gaussian one, according to the central limit theorem. We obtained expressions for the free energy and entropy with respect to the ideal gas, in terms of the potential energy moments. These can be linked to the average potential energy and its derivatives in temperature. Using thermodynamical relationships we also produce a general differential equation for the free energy as a function of temperature at fixed volume. In this paper we investigate possible exact and approximated solutions. The method was tested on a theoretical model for a solid (classical harmonic solid) and some experimental liquids. The harmonic solid has an energy distribution, which can be derived exactly from the theory. Experimental free energies of water and methanol could be reproduced very well over a temperature range of more than 300 K. For water, where the appropriate experimental data were available, also the energy and heat capacity could be reproduced very well. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470744
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  • 2
    Electronic Resource
    Electronic Resource
    Free volume properties of a simulated lipid membrane (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 9090-9099 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, an extensive analysis of free volume related properties of a lipid membrane is given. Using percolation theory, and comparing the free volume properties to those of a soft polymer, additional insights are obtained. The analyses are discussed within the framework of the four region model. It is concluded that the four regions have very different free volume properties. The region containing the dense part of the lipid tails resembles a soft polymer membrane to a large extent. The middle part of the membrane is more similar to a low density alkane. The consequences of the computed free volume properties on the permeation process of small penetrants are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471442
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  • 3
    Electronic Resource
    Electronic Resource
    Application of the quasi-Gaussian entropy theory to the calculation of thermodynamic properties of water and methane in the liquid and gas phase (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6665-6678 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this article we investigate the applicability of the statistical Gamma state as following from the quasi-Gaussian entropy theory, where all thermodynamic properties at every temperature are obtained from the knowledge of the potential energy distribution at one temperature. We compared for a typically polar system (water) and an apolar one (methane) the experimental heat capacity and entropy data with the predictions of the theory at various densities, ranging from the almost ideal gas to typical liquids. Interestingly, the behavior of water and methane is quite similar. Low-density gases and fluid-liquid systems can be described as weakly perturbed Gamma states. For intermediate densities a more complex statistical state arises. In order to describe more accurately the fluid-liquid regime, we propose in this paper a confined Gamma state, based on the division of phase-space into two different regions: one of them described by an exact Gamma state and another very unstable one. We conclude that typical fluid-liquids can be described very well by this new Gamma state approximation. We also try to give a physical interpretation of the two parts of phase space that arise from the model. The high accuracy of the theory over a large temperature range makes the approach very suitable for the prediction of thermodynamical properties at, for example, supercritical conditions. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471385
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  • 4
    Electronic Resource
    Electronic Resource
    Prediction of the liquid–vapor equilibrium pressure using the quasi-Gaussian entropy theory (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7022-7025 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We derived a method to evaluate the liquid–vapor equilibrium pressure, with high accuracy over a large range of temperature, using the quasi-Gaussian entropy theory. The final expression that we obtain for the equilibrium pressure as a function of the temperature can be considered as a very accurate approximate solution of the Clausius–Clapeyron equation. The method was applied to water, methanol and mercury, and was compared to two usual approximations of the Clausius–Clapeyron equation. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472503
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation of a charged biological membrane (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2713-2720 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A molecular dynamics simulation of a membrane with net charge in its liquid-crystalline state was carried out. It was modeled by dipalmitoylphosphatidylserine lipids with net charge, sodium ions as counterions and water molecules. The behavior of this membrane differs from that was shown by other membranes without a net charge as a consequence of strong Coulomb interaction between atoms of adjacent phospholipids. The most remarkable effect produced by such interaction between neighboring lipids is a reduction of the surface area per phospholipid compared to an uncharged membrane. In addition, other properties of the membrane were also affected by this interaction between adjacent lipids such as the atom distribution across the membrane, the diffusion coefficient of the different components of the membrane and the order parameter of the phospholipid hydrocarbon region. Some comparisons of this membrane with dipalmitoylphosphatidylcholine membrane without net charge at similar conditions are presented. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470992
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  • 6
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulations of a fully hydrated dipalmitoylphosphatidylcholine bilayer with different macroscopic boundary conditions and parameters (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 4871-4880 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We compared molecular dynamics simulations of a bilayer of 128 fully hydrated phospholipid (DPPC) molecules, using different parameters and macroscopic boundary conditions. The same system was studied under constant pressure, constant volume, and constant surface tension boundary conditions, with two different sets of charges, the single point charge (SPC) and extended single point charge (SPC/E) water model and two different sets of Lennard-Jones parameters for the interaction between water and methyl/methylene. Some selected properties of the resulting bilayer systems are compared to each other, previous simulations, and experimental data. It is concluded that in relatively high water concentration it is possible to use ab initio derived charges with constant pressure boundary conditions. The SPC water model gives a larger area per head group and a broader interface than the SPC/E model. Increasing the repulsion between water oxygens and CH2/CH3 groups has a large effect on the width of the interface and the area per head group. There is little difference between simulations with constant pressure and constant surface tension. The use of constant volume, using a reasonable estimate for the initial box dimensions, easily introduces artefacts. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472323
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  • 7
    Electronic Resource
    Electronic Resource
    Assessment of the validity of intermolecular potential models used in molecular dynamics simulations by extended x-ray absorption fine structure spectroscopy: A case study of Sr2+ in methanol solution (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9487-9497 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations have been carried out for Sr2+ in methanol using different Sr2+ Lennard-Jones parameters and methanol models. X-ray absorption fine structure (EXAFS) spectroscopy has been employed to assess the reliability of the ion-ion and ion-methanol potential functions used in the simulations. Radial distribution functions of Sr2+ in methanol have been calculated for each simulation and compared with the EXAFS experimental data. This procedure has allowed the determinations of reliable Sr2+-methanol models which have been used in longer simulations providing an accurate description of the dynamic and structural properties of this system. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476398
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  • 8
    Electronic Resource
    Electronic Resource
    On the use of the quasi-Gaussian entropy theory in noncanonical ensembles. II. Prediction of density dependence of thermodynamic properties (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 3017-3027 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In previous articles we derived and tested the quasi-Gaussian entropy theory, a description of the excess free energy in terms of the potential or full internal energy or enthalpy probability distribution, instead of the (configurational) partition function. We obtained in this way the temperature dependence of thermodynamic functions in the NVT, NpT and μVT ensembles assuming a Gaussian, Gamma or Inverse Gaussian distribution. In this article we extend the theory to describe the density dependence of thermodynamic properties, using the distribution of volume and number of particles in the isothermal-isobaric and grand canonical ensemble, respectively. In both ensembles pressure-density expressions for a Gaussian and various Gamma distributions are derived and applied to water. A Gamma description for the volume distribution turns out to be a good model in the gas range, which is in accordance with the volume distribution of an ideal gas. A Gamma description for the particle number distribution works well for liquid densities.© 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476894
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  • 9
    Electronic Resource
    Electronic Resource
    Extensions of the quasi-Gaussian entropy theory (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1893-1912 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present the quasi-Gaussian entropy theory in a comprehensive and consistent way, introducing a new derivation of the theory very suited for applications to molecular systems, and addressing its use in the case of multi-phase systems. A general derivation of the possible confinement of the system within a part of phase space is given, and for water it is shown that for this a hard sphere excluded volume model can be used. To obtain the temperature dependence of the pressure, a new differential equation is derived, and besides the previously introduced Gaussian and Gamma states, in this paper we also describe a new statistical state, the Inverse Gaussian state. We discuss the properties of these different statistical states and for water compare their thermodynamics with experimental data, finding that both the Gamma and Inverse Gaussian states are excellent descriptions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473328
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  • 10
    Electronic Resource
    Electronic Resource
    On the use of the quasi-Gaussian entropy theory in noncanonical ensembles. I. Prediction of temperature dependence of thermodynamic properties (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 3004-3016 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In previous articles we derived and tested the quasi-Gaussian entropy theory, a description of the excess Helmholtz free energy in terms of the potential energy distribution, instead of the configurational partition function. We obtained in this way the temperature dependence of thermodynamic functions in the canonical ensemble assuming a Gaussian, Gamma or Inverse Gaussian distribution. In this article we extend the theory to describe the temperature dependence of thermodynamic properties in an exact way in the isothermal-isobaric and grand canonical ensemble, using the distribution of the appropriate heat function. For both ensembles restrictions on and implications of these distributions are discussed, and the thermodynamics assuming a Gaussian or (diverging) Gamma distribution is derived. These cases have been tested for water at constant pressure, and the results for the latter case are satisfactory. Also the distribution of the heat function of some theoretical model systems is considered.© 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476893
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