  A computational method is presented to measure the spectrum of segregation
energies for an interstitial solute at grain boundaries (GBs) in a polycrystal.
For the Pd-H system, that spectrum of GB interstitial segregation energies is
found to consist of a mixture of two normal-like distributions, characteristic
of GB octahedral and tetrahedral sites. We also derive a thermodynamic isotherm
to calculate the equilibrium segregation state in a polycrystal that recognizes
the need for referencing two interstitial site types, and show that it conforms
well to experimental data in the Pd-H system.
