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: domm...@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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