  During the last two decades density functional based linear response
approaches have become the de facto standard for the calculation of optical
properties of small and medium-sized molecules. At the heart of these methods
is the solution of an eigenvalue equation in the space of single-orbital
transitions, whose quickly increasing number makes such calculations costly if
not infeasible for larger molecules. This is especially true for time-dependent
density functional tight binding (TD-DFTB), where the evaluation of the matrix
elements is inexpensive. For the relatively large systems that can be studied
the solution of the eigenvalue equation therefore determines the cost of the
calculation. We propose to do an oscillator strength based truncation of the
single-orbital transition space to reduce the computational effort of TD-DFTB
based absorption spectra calculations. We show that even a sizeable truncation
does not destroy the principal features of the absorption spectrum, while
naturally avoiding the unnecessary calculation of excitations with small
oscillator strengths. We argue that the reduced computational cost of
intensity-selected TD-DFTB together with its ease of use compared to other
methods lowers the barrier of performing optical properties calculations of
large molecules, and can serve to make such calculations possible in a wider
array of applications.
