Fig. 4.

Desorption of H₂ from Cu(110) simulated using quantum mechanical description of the atoms, both H atoms and Cu atoms. The temperature dependence of the rate constant shows the onset of tunneling at around 250 K. Inset shows how the tunneling paths (green = 0.4 eV, yellow = 0.2 eV, red = 0.02 eV) cut corners compared with the classical minimum energy path (black dashed line). HTST labels the results from classical harmonic TST calculations (black solid line), RAW-QTST labels the results from full quantum TST calculation (50, 51), and HQTST labels the results obtained from harmonic quantum TST (instanton approximation) (48). The HQTST approximation performs remarkably well. It involves much less computation than RAW-QTST and has been implemented in such a way that atomic forces from DFT calculations can be used directly (53).