The thermodynamics of the opening/closure process of a GC base pair located at the stem-loop junction of the SL1 sequence from HIV-1_Lai genomic RNA was investigated in the context of a loop-loop homodimer (or kissing complex) using molecular dynamics simulation. The free energy, enthalpy and entropy changes for the closing reaction are 0 kcal·mol^-1, -11 kcal·mol^-1 and -0.037 kcal·mol^-1·K^-1 at 300° K respectively. Furthermore it is found that the free energy change is the same for the formation of a 11 nucleotide loop closed with UG and for the formation of a 9 nucleotide loop closed with GC. Our study evidences the high flexibility of the nucleotides at the stem-loop junction explaining the weak stability of this structure.