  We describe extensions to the siesta density functional theory (dft) code
[30], for the simulation of isolated molecules and their absorption spectra.
The extensions allow for: - Use of a multi-grid solver for the Poisson equation
on a finite dft mesh. Non-periodic, Dirichlet boundary conditions are computed
by expansion of the electric multipoles over spherical harmonics. - Truncation
of a molecular system by the method of design atom pseudo- potentials of Xiao
and Zhang[32]. - Electrostatic potential fitting to determine effective atomic
charges. - Derivation of electronic absorption transition energies and
oscillator stren- gths from the raw spectra produced by a recently described,
order O(N3), time-dependent dft code[21]. The code is furthermore integrated
within siesta as a post-processing option.
