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Abstract. In this paper, we present a new algorithm, VNSOptClust, for automatic clustering. The VNSOptClust algorithm exploits the basic Variable Neighborhood Search metaheuristic to allow clustering solutions to get out of local optimality with a poor value; it considers the statistic nature of data distribution to find an optimal solution with no dependency on the initial partition; it utilizes a cluster validity index as an objective function to obtain a compact and well-separated clustering result. As an application for unsupervised Anomaly Detection, our experiments show that (i) VNSOptClust has obtained an average detection rate of 71.2% with an acceptably low false positive rate of 0.9%; (ii) VNSOptClust can detect the majority of unknown attacks from each at.tack category, especially, it can detect 84% of the DOS attacks. It appears that VNSOptClust is a promising clustering method in automatically detecting unknown intrusions.