# [gmx-users] How does fourierspacing correspond to fourier_nx, fourier_ny, and fourier_nz?

Dommert Florian dommert at icp.uni-stuttgart.de
Fri Apr 20 16:44:58 CEST 2012

On Fri, 2012-04-20 at 10:33 -0400, Andrew DeYoung wrote:
> Hi,
>
> I am wondering how the fourierspacing parameter corresponds to fourier_nx,
> fourier_ny, and fourier_nz.  The manual
> (http://manual.gromacs.org/current/online/mdp_opt.html#ewald) says, "For
> ordinary Ewald the spacing times the box dimensions determines the highest
> magnitude to use in each direction."  Will you please help me with an
> example of this?
>
> Suppose I have fourierspacing = 0.24 nm, and my box dimensions are 3.3 nm,
> 3.3 nm, and 30 nm along x, y, and z.  Then what are the values of
> fourier_nx, fourier_ny, and fourier_nz?  fourier_n* is a dimensionless
> integer, and I don't understand how we can get a dimensionless number from
> multiplying two quantities that each have dimensions of nm.  Should I
> actually divide the box dimension by fourierspacing, so that (fourier_nx,
> fourier_ny, and fourier_nz) = (3.3 nm / 0.24 nm, 3.3 nm / 0.24 nm, 30 nm /
> 0.24 nm) = (13.75, 13.75, 125) ~ (14, 14, 125)?  Or am doing this
> incorrectly?
>

This is the correct calculation. It might be that for scaling issues the
final numbers will vary a little bit, but this is not of real
importance. Actually you have to assure the error does not get too
large. 14 grid points in x and y direction sounds quite few. I would be
careful with this settings and hope they are just an example.

/Flo

> Thank you so much for your time!
>
> Andrew DeYoung
> Carnegie Mellon University
>

--
Florian Dommert
Dipl. - Phys.

Institute for Computational Physics
University Stuttgart

Pfaffenwaldring 27
70569 Stuttgart

EMail: dommert at icp.uni-stuttgart.de
Homepage: http://www.icp.uni-stuttgart.de/~icp/Florian_Dommert

Tel.: +49 - (0)711 - 68563613
Fax.: +49 - (0)711 - 68563658
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