We consider the molecular structure and energetics of extended defects in proton-disordered hexagonal ice I h. Using plane-wave density functional theory (DFT) calculations, we compute the energetics of stacking faults and determine the structure of the 30 and 90 partial dislocations on the basal plane. Consistent with experimental data, the formation energies of all fully reconstructed stacking faults are found to be very low. This is consistent with the idea that basal-plane glide dislocations in ice I h are dissociated into partial dislocations separated by an area of stacking fault. For both types of partial dislocation we find a strong tendency toward core reconstruction through pairwise hydrogen-bond reformation. In the case of the 30 dislocation, the pairwise hydrogen-bond formation leads to a period-doubling core structure equivalent to that seen in zinc-blende semiconductor crystals. For the 90 partial we consider two possible core reconstructions, one in which the periodicity of the structure along the core remains unaltered and another in which it is doubled. The latter is preferred, although the energy difference between both is rather small, so that a coexistence of both reconstructions appears plausible. Our results imply that a mobility theory for dislocations on the basal plane in ice I h should be based on the idea of reconstructed partial dislocations. © 2012 American Physical Society.852 Petrenko, V.F., Whitworth, R.W., (1999) Physics of Ice, , Oxford University Press, New YorkWang, P.K., (2002) Ice Microdynamics, , Academic, San DiegoClary, D.C., (1996) Science, 271, p. 1509. , SCIEAS 0036-8075 10.1126/science.271.5255.1509Abbatt, J.P.D., (2003) Chem. Rev., 103, p. 4783. , CHREAY 0009-2665 10.1021/cr0206418Hooke, R.L., (2005) Principles of Glacier Mechanics, , Cambridge University Press, CambridgeGillet-Chaulet, F., Durand, G., (2010) Nature (London), 467, p. 794. , NATUAS 0028-0836 10.1038/467794aSchulson, E., Duval, P., (2009) Creep and Fracture of Ice, , Cambridge University Press, CambridgeCuffey, K., Paterson, W., (2010) The Physics of Glaciers, , Butterworth-Heinemann/Elsevier, Burlington, MALouchet, F., (2004) C. R. Phys., 5, p. 687. , CRPOBN 1631-0705 10.1016/j.crhy.2004.09.001Hull, D., Bacon, D., (2001) Introduction to Dislocations, , Butterworth-Heinemann, Burlington, MAWeertman, J., Weertman, J., (1992) Elementary Dislocation Theory, , Oxford University Press, OxfordHirth, J.P., Lothe, J., (1982) Theory of Dislocations, , Wiley, New YorkDuesbery, M.S., Richardson, G.Y., (1991) C. R. Solid State Mater. Sci., 17, p. 1. , CCRSDA 1040-8436 10.1080/10408439108244630Alexander, H., Haasen, P., (1968) Solid State Physics, p. 28. , in edited by F. Seitz and D. Turnbull (AcademicBulatov, V.V., Justo, J.F., Cai, W., Yip, S., Argon, A.S., Lenosky, T., De Koning, M., Diaz Dela Rubia, T., (2001) Philos. Mag. A, 81, p. 1257. , PMAADG 0141-8610 10.1080/01418610108214440Cai, W., Bulatov, V.V., Justo, J.F., Argon, A.S., Yip, S., (2000) Phys. Rev. Lett., 84, p. 3346. , PRLTAO 0031-9007 10.1103/PhysRevLett.84.3346Glen, J., (1968) Z. Phys. B, 7, p. 43Whitworth, R.W., (1980) Philos. Mag. A, 41, p. 521. , PMAADG 0141-8610 10.1080/01418618008239330Sazaki, G., Zepeda, S., Nakatsubo, S., Yokoyama, E., Furukawa, Y., (2010) Proc. Natl. Acad. Sci. USA, 107, p. 19702. , PNASA6 0027-8424 10.1073/pnas.1008866107Martin, R.M., (2004) Electronic Structure: Basic Theory and Practical Methods, , Cambridge University Press, CambridgeKresse, G., Furthmüller, J., (1996) Comput. Mater. Sci., 6, p. 15. , CMMSEM 0927-0256 10.1016/0927-0256(96)00008-0Perdew, J.P., Burke, K., Ernzerhof, M., (1996) Phys. Rev. Lett., 77, p. 3865. , PRLTAO 0031-9007 10.1103/PhysRevLett.77.3865Kresse, G., Joubert, D., (1999) Phys. Rev. B, 59, p. 1758. , PRBMDO 1098-0121 10.1103/PhysRevB.59.1758De Koning, M., Antonelli, A., Da Silva, A.J.R., Fazzio, A., (2006) Phys. Rev. Lett., 96, p. 075501. , PRLTAO 0031-9007 10.1103/PhysRevLett.96.075501De Koning, M., Antonelli, A., Da Silva, A.J.R., Fazzio, A., (2006) Phys. Rev. Lett., 97, p. 155501. , PRLTAO 0031-9007 10.1103/PhysRevLett.97.155501De Koning, M., Antonelli, A., (2008) J. Chem. Phys., 128, p. 164502. , JCPSA6 0021-9606 10.1063/1.2902280Feibelman, P.J., (2008) Phys. Chem. Chem. Phys., 10, p. 4688. , PPCPFQ 1463-9076 10.1039/b808482nMilitzer, B., Wilson, H.F., (2010) Phys. Rev. Lett., 105, p. 195701. , PRLTAO 0031-9007 10.1103/PhysRevLett.105.195701Watkins, M., Vandevondele, J., Slater, B., (2010) Proc. Natl. Acad. Sci. U.S.A., 107, p. 12429. , PNASA6 0027-8424 10.1073/pnas.1001087107Watkins, M., Pan, D., Wang, E.G., Michaelides, A., Vandevondele, J., Slater, B., (2011) Nat. Mater., 10, p. 794. , 1476-1122 10.1038/nmat3096Hayward, J.A., Reimers, J.R., (1997) J. Chem. Phys., 106, p. 1518. , JCPSA6 0021-9606 10.1063/1.473300Li, J., (2003) Modell. Simul. Mater. Sci. Eng., 11, p. 173. , MSMEEU 0965-0393 10.1088/0965-0393/11/2/305Kantorovich, L.N., (1999) Phys. Rev. B, 60, p. 15476. , PRBMDO 1098-0121 10.1103/PhysRevB.60.15476Bulatov, V.V., Cai, W., (2006) Computer Simulations of Dislocations, , Oxford University Press, OxfordFukuda, A., Hondoh, T., Higashi, A., (1987) J. Phys. Colloques, 48, p. 1Kuhs, W.F., Bliss, D.V., Finney, J.L., (1987) J. Phys. Colloq., 3, p. 631Murray, B.J., Bertram, A.K., (2006) Phys. Chem. Chem. Phys., 8, p. 186. , PPCPFQ 1463-9076 10.1039/b513480cRaza, Z., Alfe, D., Salzmann, C.G., Klimes, J., Michaelides, A., Slater, B., (2011) Phys. Chem. Chem. Phys., 13, p. 19788. , PPCPFQ 1463-9076 10.1039/c1cp22506eBulatov, V.V., Yip, S., Argon, A.S., (1995) Philos. Mag. A, 72, p. 453. , PMAADG 0141-8610 10.1080/01418619508239934Bennetto, J., Nunes, R.W., Vanderbilt, D., (1997) Phys. Rev. Lett., 79, p. 245. , PRLTAO 0031-9007 10.1103/PhysRevLett.79.245Valladares, A., Petford-Long, A.K., Sutton, A.P., (1999) Philos. Mag. Lett., 79, p. 9. , PMLEEG 0950-0839 10.1080/095008399177606