Through the menu "Spectra - Interpolate spectrum from grid", iSpec can interpolate synthetic spectra using one grid. By default, the iSpec distribution includes a light grid named "SPECTRUM_MARCS.GES_GESv5_atom_hfs_iso.480_680nm_light" which should be used only for educational/testing purposes since it is too sparse. For scientific cases, denser grids can be downloaded from:
Grid names are formed by the radiative transfer code used, the model atmosphere, the atomic line list and the wavelength range. To use downloaded grid, they should be decompressed into the "iSpec/input/grid/" directory (e.g., "iSpec/input/grid/SPECTRUM_MARCS.GES_GESv5_atom_hfs_iso.480_680nm") and the visual interface needs to be restarted (unless they are going to be used from python scripts).
Apart from the grid, the astrophysical parameters, iSpec ask for other parameters like the projected rotational velocity (\(V\sin(i)\)), micro/macroturbulence, resolution and wavelength range/steps.