Studied as an undergraduate at the Autonomous University of Barcelona (Spain), major in Chemistry, followed by a joined PhD in physical chemistry at UC Berkeley and Autonomous University of Barcelona, with Professors Josep M. Lluch, Miquel Moreno and William H Miller (November 1999 thesis defense). Then Prof Guallar moved to NYC for a postdoc (2000-2003) at Columbia University with Prof. Richard Friesner. After NYC, he got a teanured position as an assistant Professor at Washington University in St. Louis, School of Medicine, Biochemistry & Molecular Biophysics (2003-2006). In 2006 he was appointed ICREA Research Professor in the Life Science Department at the Barcelona Supercomputing Center (BSC).