local connectivity, and fully optimized in the gas phase, with the BE of each ATOMIC AND MOLECULAR PHYSICS, atom calculated. Three different patterns of intermolecular interactions, DILUTE MATTER, UNIVERSE SCIENCE leading to three possible dimer configurations, have been identified: a SYNCHROTRON SOLEIL HIGHLIGHTS 2020 bidirectional and symmetrical H-bond (dimer1 ), a monodirectional and asymmetrical H-bond (dimer2) and a stacked dimer (dimer3). The formation of larger clusters is driven by the anisotropic distribution of H-bond donor PLEIADES BEAMLINE and acceptor sites and modulated by weaker dispersion forces between the π-conjugate charge distributions. The calculated average values of Associated publication the BEs (Fig.1 a-c) show that a good agreement with the experiments is already achieved for the dodecamer. Thus, considering that all the Unravelling molecular interactions in intermolecular and side interactions among the three stacks of molecules uracil clusters by XPS measurements are active, the dodecamer can be considered a “representative” model assisted by ab initio and tight-binding of our cluster distribution. simulations. G. Mattioli, L. Avaldi, P. Bolognesi, To shed light on the intra- and intermolecular contributions to the observed J. D. Bozek, M. C. Castrovilli, shift, calculated charge difference-density maps (Fig.2), have been used. J. Chiarinelli, A. Domaracka, For the C2 atom of the tetramer in planar configuration, for example, one S. Indrajith, S. Maclot, A. R. can see in Fig.2(a) that: (i) the major contribution to the screening comes Milosavljević, C. Nicolafrancesco, from an intramolecular contraction of the charge density (blue arrow); (ii) C. Nicolas, P. Rousseau. the surrounding molecules participate to the screening with an isotropic Scientific Reports, 10: art.n° 13081. polarization (magenta arrows) around the hole, with the first neighbors (2020). (full arrows) more involved than the second ones (dashed arrows). Fig.2(a) shows a cooperative charge redistribution but does not explain why the References four C2 atoms in the tetramer have different BEs. To illustrate this effect, [1] V. Feyer, et attributed to intermolecular interactions, and to filter out the intramolecular al. , J. Phys. Chem. A contribution, a different type of map, shown in Fig.2(b), is used. This map 113, 5736 (2009). [2] R. F. Stewart & L. Jensen, Acta shows that the screening charge is effectively moved from the peripheral Cryst. 23, 1102 (1967). regions of molecules B, C and D (blue arrows) towards the regions adjacent to the ionized molecule in an anisotropic manner, mainly populating the Corresponding author regions around the O atoms of carbonyl groups (red arrows). In order to validate the connectivity obtained by the proposed bottom-up approach, Lorenzo Avaldi finite-temperature molecular dynamics simulations of a clusters formed in CNR-Istituto di Struttura della Materia the gas phase by randomly assembling n molecules (n=12/home/webapps/asp_fr/data/asp/publications/synchrotron-soleil/synchrotron-soleil-2020/soleil-highlights-2020-hd-ss-tc50) has been Area della Ricerca di Roma 1, CP 10, 00015 Monterotondo Scalo, performed. As shown in Fig.1 the BEs for the randomly oriented 12-molecules ITALY cluster are already in good agreement with the crystallographic ones. FIGURE 2 Captions FIGURE 1: Top right and bottom panels: the structures of the uracil dimers (see text) and clusters considered in the calculations. Panels (a-c): C, N and O1s XPS spectra of the uracil monomer (top) and clusters (bottom).The bars are the average BEs predicted by the DFT theory going from the monomer (blue) to the dodecamer (purple) obtained from the crystallographic structure and for a cluster of 12 molecules obtained by a random distribution (grey). FIGURE 2: Charge difference-density plots for the ionization of the C2 atom (indicated by the yellow star) in a tetramer of planar configuration. In (a) the electron density of the whole cluster containing the core-ionized molecule is subtracted from the one of the neutral CONCLUSIONS system, while in (b) the electron density of the core- ionized molecule (A) plus the one of the other three (B, C, The modeling of the XPS spectra of uracil clusters with a bottom-up D) molecules in their ground state is subtracted from the approach based on the crystallographic structure has i) achieved a electron density of the whole cluster containing the core- good representation of the experiments, ii) allowed to disentangle the ionized molecule. In blue the zones with charge depletion while in red the ones of charge accumulation. contribution of the different intermolecular interactions for increasing cluster size, iii) shown the relevance of the H-bonds between the C = O and NH groups to determine the cluster structure and the screening of the core hole. Therefore, this approach is proposed as a computationally sustainable method to study the properties of weakly bonded molecular clusters, provided their crystal structure is known. 65