The fragment molecular orbital (FMO) method is a quantum chemistry technique that reduces the non-linear scaling associated with most standard ab initio quantum chemistry techniques while still maintaining accuracy. FMO fragments a system into smaller disjoint subsystems and then performs an ab initio calculation on each of the subsystems in the Columbic field of the other fragments, thereby avoiding an expensive calculation on the entire system. FMO has been interfaced with molecular dynamics (MD) to create FMO-MD and many applications of FMO-MD have since been published. Owing to the use of an incomplete FMO energy gradient, the original implementation of FMO-MD lacked energy conservation for the microcanonical ensemble. The origin of the incomplete FMO energy gradient and recent improvements to make the FMO energy gradient analytic are discussed. It is shown that the use of the improved energy gradient in FMO-MD results in better energy conservation for the microcanonical ensemble.
