Semiquantum versus quantum methods for grazing-incidence fast-atom diffraction: Influence of the wave-packet size

To take full advantage of the capabilities of grazing-incidence fast-atom diffraction (GIFAD) as an experimental technique for analyzing surfaces and phenomena that occur on them, versatile theoretical tools are needed that accurately describe the experiments while allowing a simple but meaningful interpretation at a reasonable computational cost. During the last years, the semiquantum method named surface initial value representation (SIVR) has been postulated to fill this room. However, to date, SIVR has not yet been validated using full quantum calculations as a reference. Here, we have contrasted GIFAD simulations performed with the SIVR approach with those obtained with the full quantum method known as multiconfiguration time-dependent Hartree (MCTDH), taking into account the influence of the size of the initial wave packet. Our comparative study, using GIFAD for the He-LiF(001) system as a benchmark, shows a very good agreement, both qualitative and quantitative, between SIVR and MCTDH simulated diffraction spectra, under different incidence conditions. These findings support the use of SIVR as a versatile theoretical tool to extract as much accurate information as possible from GIFAD experiments.