The experimental control of silver nanoparticle shape in citric acid solution is rationalized by ab initio calculations. The approximate three-fold symmetry of the acid matches that of Ag(1 1 1) and results in four silver-oxygen bonds. In contrast, citric acid forms only two bonds with Ag(1 0 0) because of the geometry mismatch. Migration of a hydrogen atom within citric acid activates the electrons of the carboxyl oxygens and provides additional binding affinity towards Ag(1 1 1). The preferential binding energy of citric acid to Ag(1 1 1) promotes crystal growth along the Ag(1 0 0) surface. (C) 2008 Elsevier B.V. All rights reserved.