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

[Andrew DeYoung]

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? 

Thank you so much for your time! 

Andrew DeYoung
Carnegie Mellon University