KtJet Atlfast now has a new Clustering scheme in addition to Cone. ClusterKtStrategy uses libraries from the Reconstruction/KtJet package. Read more about KtJet at www.ktjet.org The Kt scheme can be selected by setting ClusterMaker.Strategy = "Kt". It should be noted that the Atlfast jet smearing was based on studies using the cone algorithm, so if it is used in conjunction with Kt clustering the results cannot be guaranteed to be accurate. Future smearers that are being developed which smear cells before clustering will solve this problem. The Kt clustering algorithm has an inherent cubic time dependence on the number of input particles so is much slower than the Cone algorithm for high multiplicity events. The KtJet library has had a large amount of work done on it to improve speed performance, but it still runs much slower than the cone algorithm. It is definitely recommended to use the optimised build if selecting the Kt-Algorithm. The ratio of average run time of the ClusterMaker algorithm with the Cone and Kt strategies were calculated. This was computed for both debug and optimised builds for a typical bbbar process. debug Kt/Cone 150.5 optimised Kt/Cone 120.4 The overall effect on running time for the whole Atlfast job including Generators debug Kt/Cone 6.6 optimised Kt/Cone 3.4