EWALD

NAME
EWALD - Determine if standard Ewald or particle mesh Ewald sum must be used

SYNOPSIS
EWALD off
EWALD ON   alphal   rkcut
EWALD PME  alphal  ftt1  ftt2  ftt3  order
EWALD REMOVE_MOMENTUM

DESCRIPTION
As described in Sec. 4.1 ORAC can handle the electrostatic interactions by Ewald summation. If the argument to the command is on followed by $alphal$ and rkcut, standard Ewald is used with $\alpha = {\it alphal}$ in Å$^{-1}$ and the cutoff in k-space $k_{cut} = {\it rkcut}$ in Å$^{-1}$. The output will show what degree of convergence has been reached showing the numerical value of ${\rm erfc} \left ( r_{\rm cut} \alpha \right ) / r_{\rm cut}$ and of $\exp \left (- k_{\rm cut}^2/ 2 \alpha \right ) / k_{\rm cut}$. To chose instead PME the argument pme ${\it alphal~ ftt1~ ftt2~ ftt3~ order}$ must be chosen. Here, alphal has the same meaning as before, while order is the order of the spline and ${\it fft1~ fft2~ fft3}$ define the three dimensional grid in direct space providing the number of bins along the $a$, $b$, $c$ crystal axis, i.e. the dimensions of the 3-D FFT used in the PME method. For best efficiency ${\it fft1}$, fft2, and fft3 must be multiples of $2,3$ or $5$. Of course, if the argument is off no Ewald summation is used for the electrostatic interactions. When using PME, Newton third law is not obeyed exactly but to the numerical accuracy of the interpolation. This leads to a small momentum of the MD cell which can be removed by specifying the argument REMOVE_MOMENTUM

EXAMPLES

1

EWALD on 0.4 2.0
Standard Ewald is used with parameters $\alpha = 0.4$ Å$^{-1}$ and $k_{cut} = 2.0$ Å$^{-1}$.

2

EWALD pme 0.4 45 32 45 6
EWALD REMOVE_MOMENTUM
The electrostatic interaction is handled by PME with $\alpha = 0.4$ Å$^{-1}$, the order of the spline is $6$ and the and the number of bins for defining the grid are 45,32,45 along the $a$, $b$, $c$ crystal axis, respectively. Typically, acceptable relative accuracy ($10^{-4}$-$10^{-5}$) on electrostatic energies and forces is obtained with a grid spacing of about 1-1.2 Å along each dimension. In this example the second invocation of the EWALD command is used in order to remove the linear momentum of the MD cell.