1 A first-principles investigation of hydrous defects and IR frequencies 1 in forsterite: The case for Si vacancies 2 Koichiro Umemoto 1 , Renata M. Wentzcovitch 2,3 , Marc Hirschmann 1 , 3 David L. Kohlstedt 1 , and Anthony C. Withers 1 . 4 1 Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive 5 SE, Minneapolis, MN 55455, USA 6 2 Department of Chemical Engineering and Materials Science, University of Minnesota, 7 421 Washington Ave SE, Minneapolis, MN 55455, USA 8 3 Minnesota Supercomputing Institute, University of Minnesota, 117 Pleasant Street SE 9 Minneapolis, MN 55455 USA 10 11 12 Abstract 13 We investigate charge-balanced hydrous magnesium and silicon defects ((2H) X Mg , 14 (4H) X Si ) by first principles. Two new lowest-energy hydrogen configurations are 15 proposed for (4H) X Si . With these new configurations, the distribution of OH-stretching 16 phonon frequencies in Group I (> 3450 cm -1 ) are better reproduced. Substitution of 17 silicon with four hydrogens gives rise to significant elongation of distances between 18 oxygens at the tetrahedron of the silicon vacancy. Our calculations indicate that the 19 correlation between O-O distances and O-H-stretching phonon frequencies, which has 20 been well established for hydrous minerals, do not apply directly to nominally 21 anhydrous minerals and should not be used to determine the identity of hydrous defects 22 responsible for infrared absorption peaks. 23