--- trunk/ssdePaper/nptSSD.tex 2004/02/04 20:23:20 1021 +++ trunk/ssdePaper/nptSSD.tex 2004/02/04 22:13:36 1022 @@ -235,8 +235,8 @@ at the cutoff radius) and as a result we have two repa We have also performed a companion set of simulations {\it without} a surrounding dielectric (i.e. using a simple cubic switching function -at the cutoff radius) and as a result we have two reparamaterizations -of SSD which could be used either with or without the Reaction Field +at the cutoff radius), and as a result we have two reparamaterizations +of SSD which could be used either with or without the reaction field turned on. Simulations to obtain the preferred density were performed in the @@ -254,12 +254,11 @@ traditional quaternion integration.\cite{Evans77,Evans symplectic splitting method proposed by Dullweber {\it et al.}\cite{Dullweber1997} Our reason for selecting this integrator centers on poor energy conservation of rigid body dynamics using -traditional quaternion integration.\cite{Evans77,Evans77b} While quaternions -may work well for orientational motion under NVT or NPT integrators, -our limits on energy drift in the microcanonical ensemble were quite -strict, and the drift under quaternions was substantially greater than -in the symplectic splitting method. This steady drift in the total -energy has also been observed by Kol {\it et al.}\cite{Laird97} +traditional quaternion integration.\cite{Evans77,Evans77b} In typical +microcanonical ensemble simulations, the energy drift when using +quaternions was substantially greater than when using the symplectic +splitting method (fig. \ref{timestep}). This steady drift in the +total energy has also been observed by Kol {\it et al.}\cite{Laird97} The key difference in the integration method proposed by Dullweber \emph{et al.} is that the entire rotation matrix is propagated from @@ -449,7 +448,7 @@ results.\cite{Gillen72,Mills73,Clancy94,Jorgensen01} mean-square displacement as a function of time. The averaged results from five sets of NVE simulations are displayed in figure \ref{diffuse}, alongside experimental, SPC/E, and TIP5P -results.\cite{Gillen72,Mills73,Clancy94,Jorgensen01} +results.\cite{Gillen72,Holz00,Clancy94,Jorgensen01} \begin{figure} \begin{center} @@ -457,7 +456,7 @@ and Experimental data [Refs. \citen{Gillen72} and \cit \epsfbox{betterDiffuse.epsi} \caption{Average self-diffusion constant as a function of temperature for SSD, SPC/E [Ref. \citen{Clancy94}], TIP5P [Ref. \citen{Jorgensen01}], -and Experimental data [Refs. \citen{Gillen72} and \citen{Mills73}]. Of +and Experimental data [Refs. \citen{Gillen72} and \citen{Holz00}]. Of the three water models shown, SSD has the least deviation from the experimental values. The rapidly increasing diffusion constants for TIP5P and SSD correspond to significant decrease in density at the @@ -919,12 +918,21 @@ performed at the STP density for each of the respectiv vector can be calculated from an exponential fit in the long-time regime ($t > \tau_l^\mu$).\cite{Rothschild84} Calculation of these time constants were averaged from five detailed NVE simulations -performed at the STP density for each of the respective models. Again, -SSD/E and SSD/RF show improved behavior over SSD1 both with and -without an active reaction field. Numbers published from the original -SSD dynamics studies appear closer to the experimental values, and we -attribute this discrepancy to the implimentation of an Ewald sum -versus a reaction field. +performed at the STP density for each of the respective models. It +should be noted that the commonly cited value for $\tau_2$ of 1.9 ps +was determined from the NMR data in reference \citen{Krynicki66} at a +temperature near 34$^\circ$C.\cite{Rahman73} Because of the strong +temperature dependence of $\tau_2$, it is necessary to recalculate it +at 298 K to make proper comparisons. The value shown in Table +\ref{liquidproperties} was calculated from the same NMR data in the +fashion described in reference \citen{Krynicki66}. Again, SSD/E and +SSD/RF show improved behavior over SSD1, both with and without an +active reaction field. Turning on the reaction field leads to much +improved time constants for SSD1; however, these results also include +a corresponding decrease in system density. Numbers published from the +original SSD dynamics studies appear closer to the experimental +values, and this difference can be attributed to the use of the Ewald +sum technique versus a reaction field.\cite{Ichiye99} \subsection{Additional Observations}