  Defects in crystalline silicon consisting of a silicon self-interstitial atom
and one, two, three, or four hydrogen atoms are studied within
density-functional theory (DFT). We search for low-energy defects by starting
from an ensemble of structures in which the atomic positions in the defect
region have been randomized. We then relax each structure to a minimum in the
energy. We find a new defect consisting of a self-interstitial and one hydrogen
atom (denoted by {I,H}) which has a higher symmetry and a lower energy than
previously reported structures. We recover the {I,H_2} defect found in previous
studies and confirm that it is the most stable such defect. Our best {I,H_3}
defect has a slightly different structure and lower energy than the one
previously reported, and our lowest energy {I,H_4} defect is different to those
of previous studies.
