We present a technique for computing by first-principles simulation the absolute desorption rate γ of adsorbate molecules from a surface for any coverage and temperature. The technique is valid when the thermal equilibration rate on the surface is faster than γ , and is based on an exact expression for γ in terms of the difference of non-configurational chemical potentials of gas-phase and adsorbed molecules. This difference is expressed in terms of a potential of mean force, which is computed

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... y constrained first-principles molecular dynamics. The technique is applied to D 2 O on the MgO(001) surface at low coverage, using the generalized gradient approximation (GGA) for exchange-correlation energy. Comparisons with experimental temperature programmed desorption data allow an assessment of the accuracy of the GGA for the adsorption of D 2 O on MgO(001).