The results of an ab initio study of the properties of the ice nanotubes in isolation or inside the single-walled carbon nanotubes (SWCNTs) are reported. We consider both the van der Waals (vdW) interactions and the atomic vibrational entropy effects and find that all the ice nanotubes having different m -polygonal (m=4-8) cross sections either in isolation or lying inside the (10,10) SWCNTs are stable. The octagonal ice nanotube is the most stable one followed by the hexagonal ice nanotube, a result in disagreement with the earlier workers. Thus, striking differences between the results of an ab initio and the empirical potential studies are observed. The ice nanotubes are coupled to the carbon nanotube only by the vdW interactions. Our analysis of the computed binding energy and the structural parameters shows that the ice tube inside the wide carbon nanotube experimentally observed by Maniwa [J. Phys. Soc. Jpn. 71, 2863 (2002)] has a hexagonal structure. The electron energy gap for the ice nanotubes lie in the ultraviolet in the energy range, 3.32-3.92 eV. The optical absorption for the isolated hexagonal and octagonal ice nanotubes appears in the ultraviolet region in the energy range, 3.7-4.0 eV. The optical absorption of the (10,10) carbon nanotube is not affected significantly by the encapsulated hexagonal and octagonal ice nanotubes with the exception that all the absorption peaks are displaced toward the low-energy side by about 0.1-0.2 eV. © 2007 The American Physical Society.