Owing to their unique structural, electronic, and physico-chemical properties, molybdenum clusters are expected to play an important role in future nanotechnologies. However, their ground states are still under debate. In this study, the crystal structure analysis by particle swarm optimization (CALYPSO) approach is used for the global minima search, which is followed by first-principles calculations, to detect an obvious dimerization tendency in Mo<sub>n</sub> (n = 2-18) clusters when the 4<i>s</i> and 4<i>p</i> semicore states (SCS) are not regarded as the valence states. Further, the clusters with even number of atoms are usually magic clusters with high stability. However, after including the 4<i>s</i> and 4<i>p</i> electrons as valence electrons, the dimerization tendency exhibits a drastic reduction because the average hybridization indices H<sub>sp</sub>, H<sub>sd</sub>, and H<sub>pd</sub> are reduced significantly. Overall, this work reports new ground states of Mo<sub>n</sub> (n = 11, 14, 15) clusters and proves that SCS are essential for Mo<sub>n</sub> clusters.