We use a library of simple symmetric multi-loop secondary structures, parametrized using a variable number of stems, length of stems, loops. We are operating within an induced fit theoretical framework of receptor-ligand binding using surface similarity as optimization function for screening a library for the best geometrically fit aptamer-insulin initial complexes. We choose the multi-stem topology because it is common in known aptamer structures, and easier to find primary sequence for than other topology types as pseudo-knots. The library consisted of 54 sequences, each described by 4 parameters: number of stems, length of the basal stem, length of stems, length of multi-loops1.