The structure-based design of novel H5N1 neuraminidase inhibitors is currently a research topic of vital importance owing to both a recent pandemic threat by the worldwide spread of H5N1 avian influenza and the high resistance of H5N1 virus to the most widely used commercial drug, oseltamivir-OTV (Tamiflu). A specific criterion used in this work for determining fully acceptable conformations of potential inhibitors is a previous experimental proposal of exploiting potential benefits for drug design offered by the '150-cavity' adjacent to the NA active site. Using the crystal structure of H5N1 NA (PDB ID: 2hty) as the starting point, in a set of 54 inhibitors previously proposed by modifying the side chains of oseltamivir, 4 inhibitors were identified using two different computational strategies (ArgusLab4.0.1, FlexX-E3.0.1) both to lower the binding free energy (BFE) of oseltamivir and to have partially acceptable conformations. These 4 oseltamivr structure-based analogues were found to adopt the most promising conformations by identifying the guanidinium side chain of Arg156 as a prospective partner for making polar contacts, but none of the modified 4-amino groups of oseltamivir in the 4 favorable conformations was found to make polar contacts with the guanidinium side chain of Arg156. Hence, the structures of two additional inhibitors were designed and shown to further lower the binding free energy of OTV relative to the previous 54 inhibitors. These two novel structures clearly suggest that it may be possible for a new substituent to be developed by functional modifications at position of the 4-amino group of oseltamivir in order to make polar contacts with the guanidinium side chain of Arg156, and thereby enhance the binding of a more potent inhibitor. Several standpoints of vital importance for designing novel structures of potentially more effective H5N1 NA inhibitors are established.