Eumelanin is the most common biological melanin, a photoprotective pigment. Its broadband absorption spectrum, with increasing intensity towards higher frequencies, ensures protection from all the relevant wavelengths present in the spectrum of sunlight, in particular from the higher energy ones.
A thorough understanding of the mechanisms giving rise to this peculiar optical behaviour of eumelanin is critical in view of the possibility of synthesizing biomimetic materials with similar properties for industrial or medical applications.
It is now generally accepted that eumelanin is a macromolecule containing 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and their various redox forms, such as indolequinone (IQ). Despite several experimental and computational studies, the detailed supramolecular arrangement of the constituent molecules in eumelanin and its relation with the observed optical properties are not yet fully clarified. Experiments indicate that the pigment is made of stacked oligomers with an interlayer distance of 3-4 Å [e.g. 1]. Several models of stacking arrangements have been proposed in the literature [2,3 and refs. therein], including a model of stacked porphyrin-like tetramers  which suggests an explanation both for the observed finite lateral size of the stacked structures and for eumelanin's ability to bind metals. The electronic and optical properties of gas phase monomeric eumelanin protomolecules have been previously studied by some of the present authors within localized bases density functional theory (DFT) and time-dependent DFT (TDDFT) ; concerning stacked systems, only stacking of up to two or three layers has been investigated so far at an ab initio level in the literature, while larger arrangements have been simulated through classical molecular dynamics methods.
In this study we aim at computing the optical properties of stacked eumelanin protomolecules (such as monomers, dimers and tetramers) within plane wave DFT and TDDFT. Working with plane wave basis sets implies a twofold advantage: it provides a “natural” way to treat several-layer stacked systems (infinite-layer ones, indeed), as periodical along the stacking direction, and it makes basis set convergence tests straightforward . This will allow us to switch from the case of an isolated protomolecule to a stacked configuration, also investigating the effect on the absorption spectrum of varying the interlayer distance and possibly the molecular species, at a reasonable computational cost.
 A. A. R. Watt et al., Soft Matter 5, 3754 (2009).
 S. Meng, E. Kaxiras, Biophys. J. 94, 2095 (2008).
 C.-T. Chen et al., Nat. Commun. 5, 3859 (2014).
 R. Cardia et al., Optics and Photonics Journal 6, 41 (2016).
 E. Molteni et al., Phys. Rev. B 95, 075437 (2017).