# 2. Geometric properties¶

There are several metrics we want you to use to pre-evaluate the performance of a material for methane storage.

We used Zeo++ to compute this properties.

*largest included sphere (Di)*, the largest sphere that can fit the largest pore of the structure*largest free sphere (Df)*, the largest spherical probe that can percolate through the material*largest included sphere along the free sphere path (Dif)*, the largest sphere that can fit along the channel where the largest free sphere was evaluated. Therefore Di = Dif if there is only one channel or if the largest pore is along the largest channel.

*internal surface area*, considering the atoms of the frameworks as spheres, and rolling a spherical probe on their surface. By convention the probe has a radius of 1.86 Angstrom, half the kinetic diameter of the nitrogen molecule. This is a purely geometrical quantity and may differ a lot with the measured BET surface (Rouquerol2007).*geometric pore volume*, considering the atoms of the frameworks as spheres, all the volume outside the spheres.*probe-occupiable pore volume*(or Connolly volume ), considering the atoms of the frameworks as spheres, all the pore volume where a spherical probe could fit. The pore volume can also be distinguished into accessible or not-accessible, if the probe can actually diffuse or not inside that space: in the second case you are expected to use blocking spheres to avoid the insertion of gas molecule in Monte Carlo algorithms.*void fraction*, fraction of pore volume over the total volume of the unit cell. If you wonder why sometimes it is called “helium void fraction”, it was a convention to use the helium atom as probe to compute (BC: NOT measure experimentally!) this quantity. However, using helium to compute the void fraction is not very physically meaningful (Ongari2017).

*number of channels*, the number of unconnected channels of the porous material where the probe can diffuse through

It is very interesting to compare these metrics to the performance of your materials for a certain application, to be able to find any correlation that could help to discover the best framework for that application. See for example Wilmer2012.