Abstract：The B3LYP/6-31+G(d,p) level of theory and the polarizable continuum model (PCM) are employed to study the binding interactions between amidinium (acetamidinium or benzamidinium) and tetrazolate anion which have two binding modes (side-on and end-on) as well as the binding interaction between amidinium (acetamidinium or benzamidinium) and formate anion. We find, in the gas phase, that the end-on acetamidinium tetrazolate is more stable with its interaction energy 3.56 kJ/mol higher than that of the side-on. The end-on benzamidinium tetrazolate is more stable with its interaction energy 3.72 kJ/mol higher than that of the side-on. This means the end-on complexes are dominant. The interaction energies of acetamidinium and benzamidinium complexes with formate are higher than those of acetamidinium and benzamidinium complexes with tetrazolate by 59.35 and 58.99 kJ/mol, respectively, indicating that the interactions between amidiniums and formate are stronger than those between amidiniums and tetrazolate. The solvent effect of DMSO decreases the interaction energies in all complexes. However, in this case, the interaction between amidiniums and tetrazolate is still weaker than that in amidinium complexes with formate.