--- trunk/ssdePaper/nptSSD.tex 2003/09/10 21:28:33 758 +++ trunk/ssdePaper/nptSSD.tex 2003/09/10 22:42:57 759 @@ -23,7 +23,7 @@ \begin{document} -\title{On the temperature dependent structural and transport properties of the soft sticky dipole (SSD) and related single point water models} +\title{On the temperature dependent properties of the soft sticky dipole (SSD) and related single point water models} \author{Christopher J. Fennell and J. Daniel Gezelter{\thefootnote} \footnote[1]{Corresponding author. \ Electronic mail: gezelter@nd.edu}} @@ -160,20 +160,21 @@ $N\log N$ calculation scaling orders for $N$ particles systems, the Ewald summation and even particle-mesh Ewald become computational burdens with their respective ideal $N^\frac{3}{2}$ and $N\log N$ calculation scaling orders for $N$ particles.\cite{Darden99} - -Up to this point, a detailed look at the model's structure and ion -solvation abilities has been performed.\cite{Ichiye96} In addition, a -thorough investigation of the dynamic properties of SSD was performed -by Chandra and Ichiye focusing on translational and orientational -properties at 298 K.\cite{Ichiye99} This study focuses on determining -the density maximum for SSD utilizing both microcanonical and -isobaric-isothermal ensemble molecular dynamics, while using the -reaction field method for handling long-ranged dipolar interactions. A -reaction field method has been previously implemented in Monte Carlo -simulations by Liu and Ichiye in order to study the static dielectric -constant for the model.\cite{Ichiye96b} This paper will expand the -scope of these original simulations to look on how the reaction field -affects the physical and dynamic properties of SSD systems. +In applying this water model in these types of systems, it would be +useful to know its properties and behavior with the more +computationally efficient reaction field (RF) technique, and even with +a cutoff that lacks any form of long range correction. This study +addresses these issues by looking at the structural and transport +behavior of SSD over a variety of temperatures, with the purpose of +utilizing the RF correction technique. Towards the end, we suggest +alterations to the parameters that result in more water-like +behavior. It should be noted that in a recent publication, some the +original investigators of the SSD water model have put forth +adjustments to the original SSD water model to address abnormal +density behavior (also observed here), calling the corrected model +SSD1.\cite{Ichiye03} This study will consider this new model's +behavior as well, and hopefully improve upon its depiction of water +under conditions without the Ewald Sum. \section{Methods} @@ -232,11 +233,11 @@ in energy conservation. There is still the issue of an nine elements long as opposed to 3 or 4 elements for Euler angles and quaternions respectively. System memory has become much less of an issue in recent times, and this has resulted in substantial benefits -in energy conservation. There is still the issue of an additional 5 or -6 additional elements for describing the orientation of each particle, -which will increase dump files substantially. Simply translating the -rotation matrix into its component Euler angles or quaternions for -storage purposes relieves this burden. +in energy conservation. There is still the issue of 5 or 6 additional +elements for describing the orientation of each particle, which will +increase dump files substantially. Simply translating the rotation +matrix into its component Euler angles or quaternions for storage +purposes relieves this burden. The symplectic splitting method allows for Verlet style integration of both linear and angular motion of rigid bodies. In the integration @@ -244,10 +245,10 @@ particle simulation shows a 7\% increase in simulation evaluations to update the rotation matrix.\cite{Dullweber1997} These matrix rotations end up being more costly computationally than the simpler arithmetic quaternion propagation. On average, a 1000 SSD -particle simulation shows a 7\% increase in simulation time using the +particle simulation shows a 7\% increase in computation time using the symplectic step method in place of quaternions. This cost is more than justified when comparing the energy conservation of the two methods as -illustrated in figure \ref{timestep}. +illustrated in figure \ref{timestep}. \begin{figure} \includegraphics[width=61mm, angle=-90]{timeStep.epsi} @@ -269,7 +270,9 @@ demonstrated. with the quaternion method showing a slight energy drift over time in the 0.5 fs time step simulation. At time steps of 1 and 2 fs, the energy conservation benefits of the symplectic step method are clearly -demonstrated. +demonstrated. Thus, while maintaining the same degree of energy +conservation, one can take considerably longer time steps, leading to +an overall reduction in computation time. Energy drift in these SSD particle simulations was unnoticeable for time steps up to three femtoseconds. A slight energy drift on the