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 system (furimazine – Nanoluciferase) derived from a shrimp called Oplophorus gracilirostri with a very high luminescence intensity[3]. The protein catalyzes an oxidation reaction of furimazine by dioxygen, which leads to a new molecule, the furimamide in its excited state. A better understanding of this bioluminescent 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[4] to comply this goal. 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 (at the molecular mechanic level MM) molecular dynamics (MD) with AMBER software. Furimamide is a flexible molecule: standard force fields such as GAFF[5] (General Amber Force Field) are unable to reproduce correctly torsion angles potential energy 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 parametrize a new force field. Then, we compute electronic transitions at a QM/MM level of theory on a set of 100 snapshots extracted from each MM trajectory. We present the obtained absorption spectra of the system with different force fields (GAFF2 and new force field). We analyse also the MD simulations and compare the influence of the initial conformation on the interaction inside the cavity and on the spectra. As a perspective, we plan to use the same methodology to simulate emission spectra.