Bioluminescence is a biological phenomenon of light production by living species with many applications[1], notably for the detection of cancer cells[2]. The present study deals with a ligand-protein bioluminescent system (furimazine-Nanoluciferase) derived from a shrimp called Oplophorus gracilirostri with a very high luminescence intensity[3]. A better understanding of this system will provide insights to tailor new devices with emission of a photon with a red color, and thus a light signal more easily detectable inside the human body. Simulations of the emission and absorption spectra are relevant tools to comply this goal[4]. We focus here on the absorption spectra of the light emitter system furimamide-Nanoluciferase. In order to take into account the protein environment and the system flexibility, we carry out classical molecular dynamics (MM) with AMBER software. Furimamide is a flexible molecule: standard force fields such as GAFF[5] (General Amber Force Field) are unable to reproduce correctly potential energy along torsion angles when compared to in vacuo DFT computations. Thus, we perform a multidimensional fit using 1D and 2D potential energy surface scans with several torsion angles of the molecule in order to parameterize a new force field. Then, we compute electronic transitions at a QM/MM level of theory on a set of 100 snapshots extracted from MM trajectory. As a perspective, we plan to use the same methodology to simulate emission spectra.