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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@8532 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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sjplimp
2012-08-06 21:19:58 +00:00
parent 93181e05b0
commit e4d3494ab2
2 changed files with 28 additions and 8 deletions
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18
doc/kspace_modify.html
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@ -120,10 +120,20 @@ This keyword gives you that option.
PPPM method to compute forces on particles given electrostatic PPPM method to compute forces on particles given electrostatic
potentials on the PPPM mesh. The <I>ik</I> approach is the default. It potentials on the PPPM mesh. The <I>ik</I> approach is the default. It
performs differentiation in Kspace, but uses 3 FFTs to transfer the performs differentiation in Kspace, but uses 3 FFTs to transfer the
computed fields back to real space. The analytic differentiation, or computed fields back to real space (total of 4 FFTs per timestep). The
<I>ad</I> approach uses only 1 FFT to transfer the computed fields back to analytic differentiation, or <I>ad</I> approach uses only 1 FFT to transfer
real space, but requires a somewhat larger PPPM mesh to achieve the the computed fields back to real space (total of 2 FFTs per timestep),
same accuracy as the <I>ik</I> approach. but requires a somewhat larger PPPM mesh to achieve the same accuracy
as the <I>ik</I> approach.
</P>
<P>IMPORTANT NOTE: Currently, only the <I>pppm</I> style supports the <I>ad</I>
option. Support from other <I>pppm</I> variants will be added later.
</P>
<P>IMPORTANT NOTE: If you run the <I>ad</I> option with a constant pressure
simulation (e.g. with <A HREF = "fix_nh.html">fix npt</A>), you may see a net
slow-down. This is because the <I>ad</I> option requires more expensive
pre-computation whenever the box size changes. This happens every
step for constant-pressure simulations.
</P> </P>
<P><B>Restrictions:</B> none <P><B>Restrictions:</B> none
</P> </P>

18
doc/kspace_modify.txt
View File

@ -114,10 +114,20 @@ The {diff} keyword specifies the differentiation scheme used by the
PPPM method to compute forces on particles given electrostatic PPPM method to compute forces on particles given electrostatic
potentials on the PPPM mesh. The {ik} approach is the default. It potentials on the PPPM mesh. The {ik} approach is the default. It
performs differentiation in Kspace, but uses 3 FFTs to transfer the performs differentiation in Kspace, but uses 3 FFTs to transfer the
computed fields back to real space. The analytic differentiation, or computed fields back to real space (total of 4 FFTs per timestep). The
{ad} approach uses only 1 FFT to transfer the computed fields back to analytic differentiation, or {ad} approach uses only 1 FFT to transfer
real space, but requires a somewhat larger PPPM mesh to achieve the the computed fields back to real space (total of 2 FFTs per timestep),
same accuracy as the {ik} approach. but requires a somewhat larger PPPM mesh to achieve the same accuracy
as the {ik} approach.
IMPORTANT NOTE: Currently, only the {pppm} style supports the {ad}
option. Support from other {pppm} variants will be added later.
IMPORTANT NOTE: If you run the {ad} option with a constant pressure
simulation (e.g. with "fix npt"_fix_nh.html), you may see a net
slow-down. This is because the {ad} option requires more expensive
pre-computation whenever the box size changes. This happens every
step for constant-pressure simulations.
[Restrictions:] none [Restrictions:] none