Hi, so im attempting to evolve a periodic array of charges into a continuous potential using an FFT in fortran 90. this is, to say the least, beyond my abilities and i need help. Problem: i have an array 'Ar( )' of N evenly distributed charges, separated by a distance dx. i am using the FFTpack (available on netlib), of which the relevant subroutine calls are: initialize the fourier wave CALL dffti (N,wsave) do forward FFT CALL DFFTF (N,R,wsave) do backward FFT CALL DFFTB (N,R,wsave) this is pretty much all i know. my main attempt was to replace N with the size of the array Ar, R with the array that contains the charges ( Ar itself) and wsave with an empty dummy array. but this seems to churn out mainly empty array elements, any help would be much appreciated.