To improve the description of a coordinate bond of the reactive force field (ReaxFF), we have developed ReaxFFcoord by explicitly incorporating the coordinate bond contribution, Ecoord, into the original ReaxFF ( Chenoweth et al. J. Phys. Chem. A 2008 , 112 , 1040 - 1053 ), in which the auxiliary functions are newly suggested to describe the plug-in behavior of lone-pair electrons from a donor atom to a vacant orbital of an acceptor atom. To validate the developed ReaxFFcoord, we tested it in various systems, including a representative coordinate bond-containing molecule, namely, carbon monoxide or ammonia borane. Although the fitting abilities of the ReaxFFcoord and original ReaxFF are similar, their molecular dynamics (MD) simulations are significantly different, where MD simulations employing ReaxFFcoord provide more realistic dynamic behaviors of atoms. It is expected that the ReaxFFcoord will significantly help ReaxFF to successfully extend its applicability to the material and biological systems, including coordinate bonds in organometallic systems.