  Being one of the most promising photovoltaic materials, the metal halide
perovskites have attracted plenty of scientific effort in improving their
optoelectronic properties and long-term stability. Both properties are highly
sensitive to the nature and the concentration of the intrinsic defects in the
perovskites. The first-principles calculation is useful to complement
experiments in studying the thermodynamics relevant to the formation of the
defects and their consequences in the optoelectronic properties. However, due
to the intrinsic complexity of the halide perovskites, i.e. organic-inorganic
hybrid composition as well as covalent-ionic bonds, diverse results were
reported using different functionals within the density functional theory
framework. Here, using the most studied perovskite, MAPbI$_3$, as an example,
we conducted a comprehensive comparison of a wide range of functionals
including GGAs, meta-GGA (SCAN), with the addition of various van der Waals
(vdW) correction schemes (D3, D3(BJ), rev-vdW-DF2 and rVV10). The results
indicate meta-GGA (SCAN) predicts consistently more accurate geometries and
energies than GGAs and the vdW correction is essential for describing the van
der Waals bonds associated with the organic cations in the hybrid perovskites
and layered precursor structures (PbI$_2$). Based on the above, the SCAN+rVV10
is suggested to be the most promising functional and is expected to improve the
accuracy when studying other halide perovskites compositions.
