A recently proposed approach, called "string method," allows us to find minimum energy pathways connecting two metastable states of a system [W. E , Phys. Rev. B 66, 052301 (2002)]. So far this approach has been only used with empirical force field parametrizations of the atomic potential energy surface or in the context of macroscopic continuum models. Here we show that the string method can be efficiently combined with first-principles molecular dynamics to provide an accurate description of chemical reaction pathways and barriers. We illustrate the first-principles string molecular dynamics by applying it to the study of a surface chemical reaction, for which extensive experimental and theoretical works are available, namely, the adsorption of H-2 on the reconstructed Si(100) surface. (C) 2004 American Institute of Physics.