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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.

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# Fortran FFT problem

Can you offer guidance or do you also need help?

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