Computationally Efficient Calculation of Charges in Metal-Organic Frameworks

As our society continues to grow and develop, the demand for energy increases. Currently, most of this energy is obtained from fossil fuels, which are dwindling, creating energy security issues. Burning of fossil fuels is also a direct cause of the large amounts of CO 2 being released to the atmosphere. Atmospheric CO 2 concentration has increased to 400 ppm in 2015 1 largely due to a 29% increase in fossil fuel-related emissions between 2000 and 2008. 2 This constitutes a major issue due to the link between the atmospheric levels of CO 2 and global warming. 3, 4 This has motivated research efforts towards the development of more environmentally-friendly sources of fuel, such as methane 5-7 and hydrogen 5 , and towards the development of CO 2 capture systems. 5 In both scenarios, a novel class of materials, metal-organic frameworks (MOFs), has the potential to offer technological solutions due to their highly tunable structures and adsorption properties for the storage of fuel gases 5 or for the capture of CO 2 5 at places such as coal plants. Metal-organic frameworks (MOFs) are porous crystalline inorganic-organic ÒhybridÓ materials. They are constructed from the assembly of metal-containing clusters called secondary building units (SBUs) with multidentate organic ligands to create a three-dimensional network 5 held together by coordination bonds. The tunability of MOFs comes from the myriad of combination possibilities of MOF building blocks, which quickly increases the number of potential MOF structures to the order of millions. 5 Since investigating such a large number of structures is unfeasible by experimental means, computational approaches where potential MOF structures are constructed and evaluated in silico has helped tremendously in accelerating the rate at which new top materials are discovered. 8-10 Naturally, this approach relies on an accurate prediction of the adsorption properties of these materials. Recently, the Snurr Research Group at Northwestern University has developed a novel computational algorithm to construct thousands of MOFs where pre-constructed building blocks are mapped onto lattice templates of diverse topologies. The generated materials have been evaluated for hydrogen and methane adsorption, where electrostatic interactions are negligible. However, assessment of these materials for CO 2 capture raises the challenge of accurately describing electrostatic interactions between CO 2 and MOF atoms, which depends on an accurate assignment of electronic charges to the different atoms constituting a given MOF. For a given MOF, this can be done by deriving electronic …