AuToGraFS: Automatic Topological Generator for Framework

Article pubs.acs.org/JPCA

AuToGraFS: Automatic Topological Generator for Framework Structures Matthew A. Addicoat,* Damien E. Coupry, and Thomas Heine School of Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) and covalent organic frameworks (COFs) are recently notable examples of highly porous polymer frameworks with a raft of potential applications. Synthesis of these compounds is modular, with “connectors” and “linkers” able to be replaced almost at will in the fabrication of isoreticular frameworks (frameworks with the same underlying topology). The range of components available to form such framework structures is vast, leading to a “combinatorial explosion” problem in predicting which framework compounds might have a set of desired properties. Computational investigations can be used in both predictive and explanatory roles in this research but rely on accurate structural models. In this work, we present our software, AuToGraFS, Automated Topological Generator for Framework Structures, and show some of its advanced functionality in “computational reticular chemistry”. AuToGraFS is linked to a fully featured force field to produce fully optimized structures of arbitrary frameworks. AuToGraFS, including a graphical user interface, is publicly available for download.

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example, only five inorganic building blocks were present in the database used for the above study), and including examples of linkers not yet incorporated into synthesized MOFs will vastly increase the size of this combinatorial “haystack”.23 Adding to the complexity of the problem, the relationships between the identities of the building blocks and the properties of the resultant framework are not always straightforward; e.g., using shorter, more rigid linkers may lead to a higher accessible surface area than the isoreticular framework with longer linkers due to reducing interpenetration,24,25 functionalizing a given linker may significantly alter the chemical and binding properties of the framework,26 and even employing the same building blocks under different conditions may produce different frameworks.27 The near-infinite ocean of possibilities for framework compounds means that the vast majority of possible frameworks will never be synthetically realized. In order to focus synthetic efforts on those frameworks most likely to be useful, computational screening of framework compounds is necessary.28 In turn, this requires a way to generate coordinates of molecular frameworks with arbitrary topologies and building blocks. Equally, computational investigations may be prompted by the synthesis of unusual framework compounds.29−31 All of these computational investigations necessarily begin with a set of coordinates. In this paper, we present our method for assembling periodic and nonperiodic framework structures and its implementation within AuToGraFS: Automated Topological Generator for Framework Structures. AuToGraFS operates both as a stand-alone program, with either GUI or commandline access, or as a module within the Python Atomistic

INTRODUCTION Metal−organic frameworks (MOFs) are a class of inorganic− organic crystalline material well-known for their high surface area and porosity.1 These properties have seen MOFs developed for uses ranging from drug delivery2 to catalysis3 to optical and chemical sensing4,5 and, in particular, to gas storage and separation.6−10 MOFs are formed by two essential components: a metal or metal oxide cluster “node” or “connector” joined by an organic linker which serves as a reservoir of electrons for the metal clusters, a location for postsynthetic functionalization,11−13 and most importantly as structural “struts” or edges in the framework.14 In covalent organic frameworks (COFs), porous aromatic frameworks (PAFs), and covalent triazine frameworks (CTFs) both roles are filled by organic components. Metal− organic polyhedra (MOPs) are a discrete analog of MOFs15 and zeolitic imidazolate frameworks16 represent a special case of a MOF, where the linkers are (substituted) imidazolate and the topology is zeolite-like. The approach of breaking down framework structures into their constituent components or secondary building units (SBUs) is well-established,17 and since the first family of isoreticular MOFs was published over a decade ago,18 the concept of reticular construction of framework compounds using a variety of chemical building blocks/SBUs has become the dominant paradigm in understanding this vast family of compounds.19−21 However, this same modular construction presents a combinatorial challenge when one seeks to design a framework with specified properties. Using only building blocks gleaned from crystal structures of known MOFs and a combinatorial procedure on which they imposed a number of limits, Wilmer et al. generated an impressive database of 137 953 hypothetical MOFs.22 It is clear that employing a larger database (for © XXXX American Chemical Society

Received: July 29, 2014 Revised: September 10, 2014

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dx.doi.org/10.1021/jp507643v | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Simulation Environment (ASE).32 Using the ASE, AuToGraFS may output structures in any ASE-supported format and employ any appropriate ASE calculator for evaluation; however, by default, AuToGraFS is coupled with the UFF4MOF33 force field to provide a rapidly optimized output structure. In all the examples that follow, structures are optimized using UFF4MOF 33 within the General Utility Lattice Program (GULP)34,35

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METHOD We begin with a database of connectors, linkers, and optionally, pillars and functional groups. Each entry in the database is stored with not only its geometry (coordinates) but also UFF4MOF atom types and a complete connectivity list, including bond orders. Both philosophically and practically, AuToGraFS is synthesis-agnostic; coupling reactions other than the well-known carboxylate reaction are implicitly supported by removing such end groups from the linkers and replacing them with dummy atoms. Dummy atoms may be of two types: those that define the geometry of the building block and consequently must disappear in framework assembly or an “optional” dummy used to define sites, mostly on metalcontaining SBUs, where solvent or counterion molecules may bind. Bond orders defined to dummy atoms are obeyed in the framework assembly process. Example building blocks are shown in Figure 1.

Figure 2. Framework model corresponding to a square lattice sql. Numbered vertices indicate bonds being formed in framework assembly; vertices tagged with a prime (′) symbol indicate that the bond is formed over a periodic boundary.

in the model is “tagged”, and these tags indicate where bonds are formed between building blocks in the framework assembly process. In the “model” building mode, building blocks are placed sequentially and each bond formed has a length equal to the sum of covalent radii of the atoms being bonded; the cell vectors are then calculated at the conclusion of the assembly process and are independent of each other. In the “topology” building mode, a model, pre-scaled according to the size of the desired building blocks, is built from the topology information, which is supplied by the either the Reticular Chemistry Structure Resource (RCSR)36 or the EPINET37 database. In the actual framework building step, all building blocks are placed “simultaneously” and bonds are formed between sametagged atoms (i.e., added to the connectivity list) regardless of the bond length and geometry. Consequently, the unit cell does not change in the building process. In this building mode, the built coordinates tend to have equal, small errors in interbuilding-block bonds throughout the entire unit cell, whereas the first option tends to accumulate errors as one adds more building blocks, such that the local quality of the geometry degrades from one corner of the unit cell to the other. This problem is particularly evident where building blocks link to each other in cycles (A → B → C → D → A).

Figure 1. Geometries of four framework building blocks: (a) wellknown paddlewheel SBU (square, optionally octahedral), (b) water molecule that may be used to cap an open metal site (point cap); (c) and (d) ditopic linkers (linear), but in (d) the nitrogen capping atoms and the bond order of one-half mean that it is intended to act as a pillar in layer−pillar MOFs. Metal atoms are blue, oxygen atoms are red, carbon atoms are gray, and hydrogen atoms are white. Purple “dummy” atoms connect to linkers, defining the shape of the SBU. Sites occupied by large purple atoms are optionally filled and may be left as open metal sites. Solid bonds count as a bond order of 1; dashed lines count as 0.5.

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IMPLEMENTATION AuToGraFS contains two structure building modes, the “model” building mode and the “topology” building mode. These function similarly to the net-based assembly and connection-based assembly in the Zeo++ software of Martin and Haranczyk;38 however, the underlying processes are significantly different. AuToGraFS extends the Python Atomistic Simulation Environment (ASE),32 and maintaining the ability to manipulate built structures using ASE scripts is a key feature of AuToGraFS. Consequently, structures may be output in any format understood by the ASE or immediately calculated using any ASE calculators. Additionally, AuToGraFS is closely integrated with UFF4MOF33 and, by default, all structures are optimized using UFF4MOF in the General Utility Lattice

Framework coordinates are assembled from any valid combination of database entries, or user-supplied coordinates, by using a model. A model consists of a given number of “slots”, which are defined by their shape, center-of-mass, and the location of their corners (i.e., orientation of the slot). A straightforward layer model, sql, is illustrated in Figure 2. Each building block is translated such that its center of vertices is coincident with the centroid of the model “slot” and the building block is then rotated to align the vertices. Each vertex B



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Program (GULP).34,35 UFF4MOF was chosen as the underlying “first-pass” optimization method for the (hypothetical) structures produced within AuToGraFS, as it is a general force field, covers the entire periodic table, and has been shown to accurately reproduce structures of a broad variety of known MOF structures.33 In order to fully support molecular mechanics calculations, the Atoms datatype contained in the ASE was extended to include MMTypes and bondlists, a per atom dictionary of connectivity and bond orders. Further extensions to the Atoms datatype exist for book-keeping purposes, the most important of which are a fragment number, which permits direct manipulation of individual building blocks and functional groups even after the framework has been assembled and the original (i.e. per-SBU) index, such that individual atoms may be manipulated or replaced. Once the desired framework coordinates and connectivity have been constructed, they may be used as is or passed to an optimizer. Additionally, AuToGraFS has the capability to manipulate the structure in “chemically sensible” ways: Postbuilding Functionalization. A key feature, and in line with the philosophy of providing the user with complete freedom (but giving sensible defaults!), is that building a structure using a topology also produces an input file and model file that can recreate the structure. Using these files, a user can readily create a given structure and then functionalize, e.g., 50% of linkers. Per-Building Block Manipulation. Secondly, remembering that the ASE provides an interface to script the manipulation of structures, is the ability to manipulate substructures. For example, in assembling a square planar framework, it is generally desired that the ditopic linkers remain planar, yet when the same topology is employed, but with a paddlewheel connector, as in the SURMOF series,11,39,40 the same ditopic linker should be aligned with the metal−metal axis of the paddlewheel. AuToGraFS produces the correct result in both of these situations and provides the additional ability to rotate any individual building block by an arbitrary angle, or to reflect it about a shape-appropriate axis. Summary of Method. The steps followed by AuToGraFS may be summarized as follows. In the case of building from a framework model, AuToGraFS begins at step 4 below. These steps are illustrated for the soc topology in the Supporting Information.

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CASE STUDIES Building Block Distortion. A “challenge” for a general framework structure generator is how well the generator copes with distortion in the individual building blocks. It would be a demanding task to distort each building block, and indeed such a task seems to defeat the purpose of a general-purpose framework coordinate generator, particularly in the realm of predicting hypothetical framework structures. There are two cases to consider, differentiated by the degree of distortion present in the building blocks: Mild distortion of building blocks may occur as a result of Jahn−Teller distortion or to satisfy symmetry (space group) constraints. As an example of such relatively mild distortion, the soc topology,41,42 is informative (Figure 3). The unit cell

Figure 3. (a) Model used to generate frameworks with soc topology. Trigonal prism slots are shown in blue; rectangle slots are shown in red. Inter-building-block bonds are indicated with dashed lines. (b) Optimized structure of Al-socMOF (red) superimposed on the X-ray crystal structure of In-socMOF,42,43 counterions of both structures removed for clarity.

contains eight 6-coordinate [M3O(CO2R)]n+ building blocks joined by 12 4-coordinate linkers. Each building block distorts such that the periodic structure is composed of alternating “inward distorted” and “outward distorted” cavities. create this distortion, the linkers deviate from planarity by 7.6°, as measured from the carboxylate carbon atoms in the crystal structure of In-socMOF (which contains eight NO3 counterions, which statistically occupy 50% of the positions above the central oxygen atom of the trimeric building block).41 Similarly, in this structure the [In3O(CO2R)]+ building block has only C3 symmetry, far less than the maximum possible D3h symmetry. This level of distortion, being quite mild and dependent on the exact building blocks employed and the presence or absence of counterions, solvent, or other guest molecules, is simply ignored in the building stage and left for the optimizer to handle appropriately. A more complex example, involving strong distortion of the linkers and crossing edges, is the chs1 net, discovered by Frahm et al. to describe their UHM-7 MOF.44 The net specification is available in the personal TOPOS45,46 database. In this structure, the V-shaped organosilicon linker, 5,5′-[4,4′(dimethylsilanediyl)bis(1,4-phenyl)bis(ethyne-1,2-diyl)]diisophthalic acid, is present in two conformers: a Cs conformer occurs eight times in the unit cell and has all four linker atoms in the same plane and arms spanning 120°. The second conformer occurs 16 times, has C1 symmetry, and spans only 97°. Both conformers of the linker are abstracted as a pair of 3connected nodes, rather than one 4-connected node, and as

1. Read topology information and tag all edges, making note of those that extend over periodic boundaries. 2. Scale topology according to the size of the desired framework components. 3. Create a model with appropriate “slots” for each building block. 4. Place building blocks in the slots, via appropriate rotation and translation. 5. Update the global bonding list to account for interbuilding-block bonds and delete dummy atoms. 6. (Optional) If desired, functionalize individual building blocks. 7. Output structure in the format desired by the user (any format supported by the ASE) or return an Atoms object to the user’s ASE script. 8. Optimize the structure using UFF4MOF and undertake desired property analysis on the optimized structure. C



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there are two topologically different paddlewheels, this results in a (3,3,4,4)-connected net with I4/mmm crystallographic symmetry. This net, therefore, implies decomposing the V-shaped linker into two components, each connecting to its other half as well as two other framework components. However, because the linker is centrosymmetric, it is not possible to split it into two equal parts. In order to reassemble the entire V-shaped linker, a pseudolinear linker, corresponding to the SiX2Y2 moiety, needs to be placed between each pair of 3-connected nodes. This decomposition is shown below in Figure 4. A further difficulty

far from the desired 120° or 97°. To create the correct geometry, the positions of the Si atoms in UHM-7 were added to the net specification, lowering the space group to I4/m, the space group of UHM-7. The resultant structure is included in the Supporting Information. Final lattice parameters for the calculated structure are a = b = 29.495 Å, c = 35.932 Å, which overestimate the experimental44 lattice parameters a = b = 28.7527 Å, c = 35.4316 Å by 2.5 and 1.5%, respectively. Examples of Well-Known MOFs. MIL-53. MIL-5347 and analogous structures are composed of infinite chains of corner sharing metal octahedra. O’Keeffe and Yaghi deconstruct this MOF as MO6 octahedra and ditopic linkers forming the sra net.19 However, although this description is topologically correct, to build a framework structure using this recipe is difficult as the ditopic linkers are shared between two MO6 octahedra and the connection point is correspondingly midway between the two M atoms. In order to account for this linkage, a somewhat modified connector is employed, as shown in Figure 6 below. The topology, shown in Figure 6, then belongs

Figure 4. Decomposition of the V-shaped organosilicon linker (top) into three molecular fragments (center) and two 3-connected nodes plus a linear node (bottom). X indicates connections to a paddlewheel in UHM-7; Y indicates a connection internal to the linker.

Figure 6. Connector geometry employed to generate MIL-53 analogues. The metal atom is colored brown, oxygen atoms are red, carbon atoms are blue, purple “dummy” atoms connect to linkers, and white “dummy” atoms connect to the adjacent connector. A further dummy atom/connection point may be placed on the central oxygen atom, to attach a capping ligand, but it is not shown here. The semitransparent octahedron centered on the metal atom shows that the octahedral coordination of the metal atom is maintained.

in appropriately assembling the linker arises in that using the midpoints of the edges connecting paris of 3-connected nodes (light and dark red in Figure 5 below) forces the linker components to be placed in a coplanar (or nearly so) geometry,

to the Imma space group and connects four MO6 octahedra with four ditopic linkers. Employing the monomer above with the metal atom Cr, and a ditopic “benzene” linker and bare bridging oxygen atoms (Figure 7), the resultant lattice parameters are shown in Table 1. The largest error is an 8% underestimation of the a lattice vector, which is most likely due to the bare oxygen atom that bridges pairs of Cr atoms. The errors on the b and c vectors are less than 5%. The MIL structures are well-known to be sensitive to their environment, in particular the presence of solvent, and so moderate deviation from experimental lattice vectors is expected for a “bare” MIL53 structure. mtn-e Topology. MIL-101 is a well-known complex MOF that possesses the mtn-e topology.49 It has a cubic unit cell, with a cell dimension of a = 88.9 Å; the unit cell contains a total of 272 trimeric SBUs and 816 ditopic linkers, leading to 1088 distinct objects. Building this topology with a D3h-symmetrized Al trimeric oxo-centered SBU and a ditopic benzene linker (i.e.,

Figure 5. View of the chs1 net: (a) along the a-axis; (b) along the caxis. 4-connected nodes are blue, and 3-connected nodes are red. In both cases, topologically different nodes are shown using light and dark colors. Edge midpoints are shown as small black spheres. D



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Figure 7. Unit cell used to generate MIL-53 analogues. The four ditopic linkers (blue cylinders) and four connectors (red octahedra) shown in dark color are the eight objects placed by AuToGraFS. Periodic images are shown in pale blue and red.

Table 1. AuToGraFS Produced and Experimental Lattice Parameters for MIL-53(Cr) (All Angles, α, β, γ = 90°) lattice parameter

calculated

experiment (ref 48)

% error

a b c

6.213 16.124 13.231

6.812 16.733 13.038

−8.8 −3.6 1.5

Figure 8. snf-net model of MFU-4l showing the dummy atoms (red crosses) used to link the framework structure (red crosses). The model is shown with the bonds between individual building blocks slightly expanded for clarity.

creating the Al-analogue of MIL-101) results in a structure with a unit cell dimension of a = 90.2 Å, an overestimation (with respect to the chromium-based structure of Ferey et al.48) of only 1.5%. Upon optimization (without capping ligands), this dimension shrinks to a = 88.0 Å. Building an analogous structure, using a Cr trimeric oxo-centered SBU and a ditopic biphenyl linker, yields a starting structure with a cell dimension of a = 126.1 Å, which reduces to 120.5 Å after optimization, again without the presence of counterions/capping ligands. MFU-4. MFU-450 and MFU-4l(arge)51 have been investigated for separation of H2/D2 mixtures52 and their redox properties.53 The unit cell of these frameworks contains eight [MoctM4tetCl4(ta)6], “Kuratowski-type” SBUs, each doubly connected to three others within the cell via “benzene” (MFU-4) and “oxanthrene” linkers. The structure bears a strong resemblance to the primitive cubic lattice, pcu net; however, the multiple connectivity of each SBU makes it less than straightforward to determine the net that this structure exemplifies, as multiple edges between the same vertices are not permitted in crystal nets.19 build this framework and analogues thereof, we therefore consider the “Kuratowski-type” SBU as a special 6-connected node and the linkers as rectangles, therefore employing the snf net. The assembled framework, including the dummy “link” atoms is shown below in Figure 8. The final MFU-4 structure has a lattice parameter a = b = c = 21.6196 Å, which is almost exactly equal to the experimental lattice constant of 21.6265 Å. For MFU-4l(arge)(Co4Zn), our optimized lattice parameters are a = b = c = 31.015 Å, which deviates less than 0.1% from the experimental value of 30.995 Å. Interpenetrated Frameworks. The approach of building framework structures using primarily the connectivity of the components is particularly useful when interpenetrated (catenated) frameworks building, where atoms may be sufficiently close together to mislead any automated guess of the connectivity of the assembled framework (as in the chs1 case described above). To illustrate the building of several interpenetrated networks from the same starting components,

the triphenylene (TP) and ditopic pyrromellitic diimide (PyrDI) linker were selected, illustrated in Figure 9.54 Note

Figure 9. Tritopic TP connector and ditopic Pyr linker as used to form the interpenetrated framework structures in Table 2.

that although, in general, AuToGraFS tries to use connectors and linkers that resemble those employed in the synthesis (capping dummy atoms notwithstanding) as much as possible, in this case it is more straightforward to include the boron atoms in the TP connector as it means forming a single B−C bond, rather than two CAr−O bonds. E



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Table 2. Optimized Parameters of Hypothetical Interpenetrated COF Structuresa RCSR net symbol

no. of atoms in unit cell

a, Å

b, Å

c, Å

α, deg

β, deg

γ, deg

density, g/cm3

surface area, m2/g

pore limiting diameter, Å

max. pore diameter, Å

eta-c eta-c3 eta-c4 lig-c srs-a-c srs-c srs-c2* srs-c3 srs-c4 srs-c4* srs-c8 ths-c twt-c twt-c3

540 540 540 720 4320 1440 360 2160 360 720 720 360 540 540

90.41 52.78 89.42 84.91 175.60 84.84 61.01 84.81 42.44 61.73 42.45 38.84 75.72 44.62

91.52 52.21 90.55 85.04 176.83 84.83 60.66 84.83 42.44 62.83 42.45 38.33 76.87 43.55

67.60 68.41 13.63 42.30 173.87 84.94 41.23 84.87 42.44 38.49 42.42 83.08 28.11 56.84

90 90 90 90 90 90 90 90 90 90 90 90 90 90

90 90 90 90 90 90 90 90 90 90 90 90 90 90

120 120 120 90 90 90 90 90 90 90 90 90 120 120

0.018 0.055 0.093 0.039 0.013 0.039 0.039 0.059 0.078 0.080 0.156 0.048 0.063 0.095

8287.66 8297.60 6131.74 8247.02 8268.00 8226.52 8270.74 8297.38 8233.77 8310.90 8327.39 8184.47 8270.28 7424.51

82.74 37.55 82.20 54.44 76.28 34.90 56.34 36.28 23.97 22.77 14.56 33.61 40.74 31.63

87.60 39.20 82.85 58.45 81.32 37.35 57.00 45.79 33.30 27.70 33.29 40.70 42.47 35.13

a Cell parameters are calculated using GULP34,35 and surface area, pore size, and gate size of the optimized structures are calculated using a cubelet size of 0.2 Å in PoreBlazer,57 except for srs-a-c, where a cubelet size of 0.5 Å was employed.

It is known that, under synthetic conditions, these two components form an slipped-AA stacked55 2D-layer COF (hcb layer topology) and it is unlikely that these two components would form any of the interpenetrated frameworks below;20,56 however, it is a useful illustrative example of the capabilities of AuToGraFS. The RCSR36 contains 15 interpenetrated nets that contain a single 3-coordinated vertex; of these, 14 were constructed (sgn-z-c was excluded because it is a rod net, with connections between the rods) and the predicted cell parameters, surface area, and pore and gate diameters calculated using PoreBlazer57 are shown below in Table 2. Figures and coordinates for all structures are contained in the Supporting Information. Nonperiodic Frameworks. Though most uses of a general framework generator are anticipated to be producing periodic structures, AuToGraFS is equally capable of producing molecular framework structures. An example of such molecular frameworks are the tetrahedral M4L4 cages synthesized by Bilbeisi et al.58 All four cage structures were built using a tetrahedral pseudotopology and a “precapped” Fe atom as shown in Figure 10. The metal−metal distances, which represent the edge length of the tetrahedron, are shown in Table 3, the agreement with the X-ray crystal structures is good. The A and B linkers are of similar size, and using these two ligands Bilbeisi et al. also synthesized several “mixed” cages, i.e., M4AmBn, m + n = 4, and so using the model file produced for

Table 3. Metal−Metal Distances of Tetrahedral M4L4 Molecular Cages ligand

rMM

rMM (ref 58)a

A B C D

11.8 11.6 14.1 18.6

11.9 11.8 ≈14 ≈18.2

a

Results taken from ref 58. In ref 58, crystal structures for the largest two ligands (C and D) were not available and so MM2 simulations were employed.

cage B, we substituted two linkers for A linkers. Optimized structures for all cages are contained in the Supporting Information.

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CONCLUSIONS In this paper we have presented AuToGraFS, the Automated Topological Generator for Framework Structures, and shown its capability for a range of straightforward to complex topologies. AuToGraFS assembles periodic and nonperiodic framework structures from topological information, and specification of individual linkers and connectors. Close integration with UFF4MOF and the Python Atomistic Simulation Environment (ASE) permits full optimization of generated structures and subsequent calculation of a broad range of properties, including pore sizes, surface area, and PXRD patterns, all within a unified simulation environment. AuToGraFS has an integrated database of common connectors and linkers, for straightforward assembly of common framework structures, through either command line or GUI interfaces. AuToGraFS may be freely downloaded from https://github.com/maddicoat/AuToGraFS. Future releases will extend the inbuilt component and topological databases.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information includes an illustrated guide to the method, figures of all interpenetrated frameworks, detailed discussion about scaling topologies, and cif files containing all structures generated in this work. This material is available free of charge via the Internet at http://pubs.acs.org

Figure 10. Connector and linker used to form a molecular cage framework. The linker shown is linker B from ref 58. F



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AUTHOR INFORMATION

Corresponding Author

*M. A. Addicoat. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the European Commission (ERC StG C3ENV GA 256962 and MC-IAPP QUASINANO, GA 251149). M.A.A. is supported by a Marie Curie Actions (MC-IIF, GA-MOF, GA 327758) fellowship. We gratefully acknowledge the assistance of Prof. Julian Gale in incorporating UFF4MOF into the GULP software package. M.A.A. also thanks Dr. Andreas Mavrandonakis for many useful discussions.

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