  The emergence of oxygenic photosynthesis was a major event in Earth's
evolutionary history and was facilitated by chlorophylls (a major category of
photopigments). The accurate modelling of photopigments is important to
understand the characteristics of putative extraterrestrial life and its
spectral signatures (detectable by future telescopes). In this paper, we
perform a detailed assessment of various time-dependent density-functional
theory (TD-DFT) methods for predicting the absorption spectra of chlorophyll a,
with particular emphasis on modern low-cost approximations. We also investigate
a potential extraterrestrial photopigment called phot0 and demonstrate that the
electronegativity of the metal ion may exert a direct influence on the
locations of the absorption peaks, with higher electronegativity inducing
blue-shifting and vice-versa. Based on these calculations, we established that
global-hybrid approximations with a moderate percentage of exact exchange --
such as M06 and PW6B95 -- are the most appropriate compromise between cost and
accuracy for the computational characterization of photopigments of
astrobiological interest. We conclude with a brief assessment of the
implications and avenues for future research.
