On the importance and challenges of modelling extraterrestrial photopigments via density-functional theory

TL;DR

This study evaluates various TD-DFT methods for modeling extraterrestrial photopigments, highlighting the influence of metal electronegativity on absorption spectra and identifying optimal computational approaches for astrobiological research.

Contribution

It provides a detailed assessment of TD-DFT methods for predicting photopigment spectra and explores how metal electronegativity affects absorption peaks, guiding future extraterrestrial photopigment modeling.

Findings

Hybrid functionals like M06 and PW6B95 offer the best cost-accuracy balance.

Metal electronegativity influences the blue-shifting of absorption peaks.

Moderate exact exchange in functionals is optimal for photopigment modeling.

Abstract

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.