--- trunk/electrostaticMethodsPaper/electrostaticMethods.tex 2006/03/20 21:40:49 2645 +++ trunk/electrostaticMethodsPaper/electrostaticMethods.tex 2006/03/21 14:58:12 2649 @@ -140,10 +140,10 @@ V_\textrm{elec} = \frac{1}{2}& \sum_{i=1}^N\sum_{j=1}^ \end{split} \label{eq:EwaldSum} \end{equation} -where $\alpha$ is a damping parameter, or separation constant, with -units of \AA$^{-1}$, $\mathbf{k}$ are the reciprocal vectors and are -equal to $2\pi\mathbf{n}/L^2$, and $\epsilon_\textrm{S}$ is the -dielectric constant of the surrounding medium. The final two terms of +where $\alpha$ is the damping or convergence parameter with units of +\AA$^{-1}$, $\mathbf{k}$ are the reciprocal vectors and are equal to +$2\pi\mathbf{n}/L^2$, and $\epsilon_\textrm{S}$ is the dielectric +constant of the surrounding medium. The final two terms of eq. (\ref{eq:EwaldSum}) are a particle-self term and a dipolar term for interacting with a surrounding dielectric.\cite{Allen87} This dipolar term was neglected in early applications in molecular @@ -159,8 +159,9 @@ convergent behavior. Indeed, it has often been observ convergence. In more modern simulations, the simulation boxes have grown large enough that a real-space cutoff could potentially give convergent behavior. Indeed, it has often been observed that the -reciprocal-space portion of the Ewald sum can be vanishingly -small compared to the real-space portion.\cite{XXX} +reciprocal-space portion of the Ewald sum can be small and rapidly +convergent compared to the real-space portion with the choice of small +$\alpha$.\cite{Karasawa89,Kolafa92} \begin{figure} \centering @@ -176,7 +177,7 @@ The original Ewald summation is an $\mathscr{O}(N^2)$ \end{figure} The original Ewald summation is an $\mathscr{O}(N^2)$ algorithm. The -separation constant $(\alpha)$ plays an important role in balancing +convergence parameter $(\alpha)$ plays an important role in balancing the computational cost between the direct and reciprocal-space portions of the summation. The choice of this value allows one to select whether the real-space or reciprocal space portion of the @@ -714,18 +715,18 @@ The althernative methods were also evaluated with thre manner across all systems and configurations. The althernative methods were also evaluated with three different -cutoff radii (9, 12, and 15 \AA). It should be noted that the damping -parameter chosen in SPME, or so called ``Ewald Coefficient'', has a -significant effect on the energies and forces calculated. Typical -molecular mechanics packages set this to a value dependent on the -cutoff radius and a tolerance (typically less than $1 \times 10^{-4}$ -kcal/mol). Smaller tolerances are typically associated with increased -accuracy at the expense of increased time spent calculating the -reciprocal-space portion of the summation.\cite{Perram88,Essmann95} -The default TINKER tolerance of $1 \times 10^{-8}$ kcal/mol was used -in all SPME calculations, resulting in Ewald Coefficients of 0.4200, -0.3119, and 0.2476 \AA$^{-1}$ for cutoff radii of 9, 12, and 15 \AA\ -respectively. +cutoff radii (9, 12, and 15 \AA). As noted previously, the +convergence parameter ($\alpha$) plays a role in the balance of the +real-space and reciprocal-space portions of the Ewald calculation. +Typical molecular mechanics packages set this to a value dependent on +the cutoff radius and a tolerance (typically less than $1 \times +10^{-4}$ kcal/mol). Smaller tolerances are typically associated with +increased accuracy at the expense of increased time spent calculating +the reciprocal-space portion of the +summation.\cite{Perram88,Essmann95} The default TINKER tolerance of $1 +\times 10^{-8}$ kcal/mol was used in all SPME calculations, resulting +in Ewald Coefficients of 0.4200, 0.3119, and 0.2476 \AA$^{-1}$ for +cutoff radii of 9, 12, and 15 \AA\ respectively. \section{Results and Discussion}