Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Efficient solution of Poisson's equation with free boundary conditions   Interpolating scaling functions give a faithful representation of a localized
charge distribution by its values on a grid. For such charge distributions,
using a Fast Fourier method, we obtain highly accurate electrostatic potentials
for free boundary conditions at the cost of O(N log N) operations, where N is
the number of grid points. Thus, with our approach, free boundary conditions
are treated as efficiently as the periodic conditions via plane wave methods.

6
The categories of this paper are: astro-ph
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The categories of this paper are: adap-org
15
The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
13
The categories of this paper are: quant-ph
9
The categories of this paper are: nlin
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
26
The categories of this paper are: bayes-an
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th