  With the advent of new synthesis and large-scale production technologies,
nanostructured gas-adsorbent materials (GAM) like carbon nanocomposites and
metal-organic frameworks are becoming increasingly more influential in our
everyday lives. First-principles methods based on density functional theory
(DFT) have been pivotal in establishing the rational design of GAM, a factor
which has tremendously boosted their development. However, DFT methods are not
perfect and due to the stringent accuracy thresholds demanded in modelling of
GAM (i.e., exact binding energies to within ~0.01 eV) these techniques may
provide erroneous conclusions in some challenging situations. Examples of
problematic circumstances include gas-adsorption processes in which both
electronic long-range exchange and nonlocal correlations are important, and
systems where many-body energy and Coulomb screening effects cannot be
disregarded. In this critical review, we analyse recent efforts done in the
assessment of the performance of DFT methods in the prediction and
understanding of GAM. Our inquiry is constrained to the areas of hydrogen
storage and carbon capture and sequestration, for which we expose a number of
unresolved modelling controversies and define a set of best practice simulation
principles. Also, we identify the subtle problems found in the generalization
of DFT benchmark studies performed in model cluster systems to real materials,
and discuss effective approaches to circumvent them. The increasing awareness
of the strengths and imperfections of DFT methods in the simulation of
gas-adsorption phenomena should lead in the medium term to more precise, and
hence even more fruitful, ab initio engineering of GAM.
