We present linear optical absorptions of photo/thermochromic molecules interacting with a gold nanoparticle. The photo/thermochromic system is the dihydroazulene/vinylheptafulvene (DHA/VHF) system and our aim is to study the effects of the interaction between the gold nanoparticle and the molecular systems. We consider the changes of the one-photon excitations of the dihydroazulene/vinylheptafulvene system as we increase the interactions between the molecules and the nanoparticle by decreasing the distance between them. We utilize a quantum mechanical/molecular mechanical method for investigating the photo/thermochromic molecular system interacting with the gold nanoparticle. The photo/thermochromic molecules are described quantum mechanically using density functional theory whereas the gold nanoparticle is represented as gold atoms with atomic polarizabillities using molecular mechanics. We observed that the optical properties of the photo/thermochromic systems are affected by the presence of the nanoparticle and the changes depend strongly on the conformer of the molecular system along with the relative orientation and distance between the photo/thermochromic molecules and the nanoparticle.