We employ DFT/B3LYP method to investigate linear open-chain clusters (n = 2-8) of the cis-triaziridine molecule that is a candidate molecule for high energy density materials (HEDM). Our calculations indicate that the pervasive phenomena of cooperative effects are observed in the clusters of it = 3-8, which are reflected in changes in lengths of N center dot center dot center dot H hydrogen bonds, stretching frequencies, and intensities of N-H bonds, dipole moments, and charge transfers as Cluster size increases. The n(N) Alpha -> sigma*(N-H) interactions, i.e., the charge transfers from lone pairs (n(N)) of the N atoms into antibonds (sigma*) of the N-H bonds acting as H-donors, can be used to explain the observed cooperative phenomena. The approaches based upon natural bond orbital (NBO) method and theory of atoms in molecule (AIM) to evaluating N center dot center dot center dot H strengths are found to be equivalent. In the process of N center dot center dot center dot H bonding, cooperative nature of n(N) -> sigma*(N-H) interactions promotes formation of stronger N center dot center dot center dot H bonds as reflected in increases in the capacities of cis-triaziridine clusters to concentrate electrons at the bond critical points of N center dot center dot center dot H bonds. The calculated nonadditive energies also show that the cooperative effects due to n(N) -> sigma*(N-H) interactions indeed provide additional stabilities for the clusters.