1D potential energy surfaces of HCOOH as function of the OH torsional angle φ₁. Bottom panel: ground S₀ electronic state. Top panel: excited S₁ state. 1D cuts were obtained from the 2D grid (see text) by setting φ₂ = 180° and φ₂ = 300° for S₀ and S₁, respectively. We also show the vibrational-wave functions obtained from a 1D model. The different geometrical structures of the HCOOH molecule in each energy minimum are shown.