  Previous calculations of the dielectric and optical properties of 2D
materials often overlooked or circumvented the influence of vacuum spacing
introduced in periodic calculations, which gave rise to mispredictions of the
intrinsic properties of 2D materials or merely qualitative results. We first
elucidate the relationship between the vacuum spacing and the dielectric and
optical properties of 2D materials in periodic calculations, and then bring
forward an effective method to accurately predict the dielectric and optical
properties of 2D materials by restoring the intrinsic dielectric functions of
2D materials independent of the additional vacuum spacing. As examples, the
intrinsic dielectric and optical properties of ultrathin h-BN and MoS2 from
monolayer to pentalayer, including dielectric functions, optical absorption
coefficients, refraction indexes, reflectivities, extinction coefficients, and
energy loss functions, have been calculated by our method. Our calculations
reveal that the out-of-plane optical dielectric constants, static refraction
indexes, and static reflectivities of 2D h-BN and MoS2 increase as the number
of layers increases, while the in-plane counterparts remain unchanged.
Excitonic frequency-dependent optical properties of h-BN and MoS2 from
monolayer to bulk are also calculated by solving the Bethe-Salpeter equation
and show strong anisotropy. In better agreement with experimental results than
previous calculations, the presented method demonstrates enormous potential to
investigate the dielectric and optical properties of other 2D materials
extensively and quantitively.
