Through a combined use of experimental and theoretical approaches such as XRPD, EXAFS, IR, and UV−vis spectroscopies and ab initio periodic DFT calculations, we report a detailed characterization of structural, vibrational, and electronic properties of UiO-66 (Zr-BDC MOF) in its hydroxylated and dehydroxylated forms. The stability of the materials with respect to the most common solvents, acids, and bases is determined by combining XRPD and TGA/MS techniques. The structures of the two forms of UiO-66 are refined through an interactive XRPD/EXAFS approach and validated by ab initio calculations. Experimental and calculated IR spectra are reported and compared to enlighten the nature of vibrational modes upon dehydroxylation and to show the complete reversibility of the dehydration/hydration phenomenon. Experimental and calculated band gaps are also reported and compared. In this work, we show the necessity to combine, in a synergic way, different experimental techniques and periodic ab initio approaches to disclose and fully understand the nature of complex novel materials such as UiO-66 on structural, vibrational, and electronic grounds. The correct structure refinement could not be possible using one of these three approaches alone, in particular, XRPD data were unable to detect an important distortion of the Zr6O6 units of the dehydrated material that was, however, foreseen in the ab initio calculations and measured in the EXAFS spectra.

The features of the publicly distributed CRYSTAL program for quantum‐mechanical condensed matter simulations are reviewed and the latest version of the code, namely CRYSTAL17, is introduced.

The capabilities of the CRYSTAL14 program are presented, and the improvements made with respect to the previous CRYSTAL09 version discussed. CRYSTAL14 is an ab initio code that uses a Gaussiantype basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first-second transition metal rows of the periodic table. A variety of density functionals is available, including as an extreme case Hartree-Fock; hybrids of various nature (global, range-separated, double) can be used. In particular, a very efficient implementation of global hybrids, such as popular B3LYP and PBE0 prescriptions, allows for such calculations to be performed at relatively low computational cost. The program can treat on the same grounds zero-dimensional (molecules), one-dimensional (polymers), two-dimensional (slabs), as well as three-dimensional (3D; crystals) systems. No spurious 3D periodicity is required for low-dimensional systems as happens when plane-waves are used as a basis set. Symmetry is fully exploited at all steps of the calculation; this permits, for example, to investigate nanotubes of increasing radius at a nearly constant cost (better than linear scaling!) or to perform self-consistent-field (SCF) calculations on fullerenes as large as (10,10), with 6000 atoms, 84,000 atomic orbitals, and 20 SCF cycles, on a single core in one day. Three versions of the code exist, serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated, whereas in the third one the matrices in reciprocal space are distributed for diagonalization. All the relevant vectors are now dynamically allocated and deallocated after use, making CRYSTAL14 much more agile than the previous version, in which they were statically allocated. The program now fits more easily in low-memory machines (as many supercomputers nowadays are). CRYSTAL14 can be used on parallel machines up to a high number of cores (benchmarks up to 10,240 cores are documented) with good scalability, the main limitation remaining the diagonalization step. Many tensorial properties can be evaluated in a fully automated way by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, electric field gradients, Born tensors and so forth. Many tools permit a complete analysis of the vibrational properties of crystalline compounds. The infrared and Raman intensities are now computed analytically and related spectra can be generated. Isotopic shifts are easily evaluated, frequencies of only a fragment of a large system computed and nuclear contribution to the dielectric tensor determined. New algorithms have been devised for the investigation of solid solutions and disordered systems. The topological analysis of the electron charge density, according to the Quantum Theory of Atoms in Molecules, is now incorporated in the code via the integrated merge of the TOPOND package. Electron correlation can be evaluated at th...

CRYSTAL [1] computes the electronic structure and properties of periodic systems (crystals, surfaces, polymers) within Hartree-Fock [2], Density Functional and various hybrid approximations. CRYSTAL was developed during nearly 30 years (since 1976) [3] by researchers of the Theoretical Chemistry Group in Torino (Italy), and the Computational Materials Science group in CLRC (Daresbury, UK), with important contributions from visiting researchers, as documented by the main authors list and the bibliography.The basic features of the program CRYSTAL are presented, with two examples of application in the field of crystallography [4,5].

The problem of numerical accuracy in the calculation of vibrational frequencies of crystalline compounds from the hessian matrix is discussed with reference to alpha-quartz (SiO(2)) as a case study and to the specific implementation in the CRYSTAL code. The Hessian matrix is obtained by numerical differentiation of the analytical gradient of the energy with respect to the atomic positions. The process of calculating vibrational frequencies involves two steps: the determination of the equilibrium geometry, and the calculation of the frequencies themselves. The parameters controlling the truncation of the Coulomb and exchange series in Hartree-Fock, the quality of the grid used for the numerical integration of the Exchange-correlation potential in Density Functional Theory, the SCF convergence criteria, the parameters controlling the convergence of the optimization process as well as those controlling the accuracy of the numerical calculation of the Hessian matrix can influence the obtained vibrational frequencies to some extent. The effect of all these parameters is discussed and documented. It is concluded that with relatively economical computational conditions the uncertainty related to these parameters is smaller than 2-4 cm(-1). In the case of the Local Density Approximation scheme, comparison is possible with recent calculations performed with a Density Functional Perturbation Theory method and a plane-wave basis set.

The B3LYP method augmented with a damped empirical dispersion term (Àf(R)C 6 /R 6 ) is shown to yield structures and cohesive energies, for a representative set of molecular crystals, in excellent agreement with experimental data. Vibrational lattice modes of crystalline urea are also reported to be very close to experiment. The role of the damping function in scaling the dispersion contribution has been analyzed as well as the relevance of the BSSE in the prediction of structure and cohesive energy.

Adsorption of carbon monoxide, dinitrogen, and carbon dioxide on the porous metal−organic framework Mg-MOF-74 was investigated by means of a combined methodology comprising variable-temperature infrared spectroscopy and ab initio periodic DFT-D calculations using the CRYSTAL code. Both CO and N2 were found to form nearly linear (Mg2+···CO and Mg2+···NN) adsorption complexes, in contrast with CO2, which forms an angular Mg2+···OCO complex. From IR spectra recorded at a variable-temperature, the standard adsorption enthalpy (ΔH 0) was found to be −29, −21, and −47 kJ mol−1 for CO, N2, and CO2, respectively. Calculated values of ΔH 0, including an empirical correction for dispersion forces, resulted to be in a reasonably good agreement with those experimentally obtained. Calculations also showed the very significant role played by dispersion forces, which account for about one-half of the adsorption enthalpy for each of the three adsorbates, CO, N2, and CO2. The results are discussed in the broader context of the adsorption of the same gases on other MOFs and also on zeolites.

The recently discovered UiO-66/67/68 class of isostructural metallorganic frameworks (MOFs) [J. H. Cavka et al. J. Am. Chem. Soc., 2008, 130, 13850] has attracted great interest because of its remarkable stability at high temperatures, high pressures and in the presence of different solvents, acids and bases [L. Valenzano et al. Chem. Mater., 2011, 23, 1700]. UiO-66 is obtained by connecting Zr(6)O(4)(OH)(4) inorganic cornerstones with 1,4-benzene-dicarboxylate (BDC) as linker resulting in a cubic MOF, which has already been successfully reproduced in several laboratories. Here we report the first complete structural, vibrational and electronic characterization of the isostructural UiO-67 material, obtained using the longer 4,4'-biphenyl-dicarboxylate (BPDC) linker, by combining laboratory XRPD, Zr K-edge EXAFS, TGA, FTIR, and UV-Vis studies. Comparison between experimental and periodic calculations performed at the B3LYP level of theory allows a full understanding of the structural, vibrational and electronic properties of the material. Both materials have been tested for molecular hydrogen storage at high pressures and at liquid nitrogen temperature. In this regard, the use of a longer ligand has a double benefit: (i) it reduces the density of the material and (ii) it increases the Langmuir surface area from 1281 to 2483 m(2) g(-1) and the micropore volume from 0.43 to 0.85 cm(3) g(-1). As a consequence, the H(2) uptake at 38 bar and 77 K increases from 2.4 mass% for UiO-66 up to 4.6 mass% for the new UiO-67 material. This value is among the highest values reported so far but is lower than those reported for MIL-101, IRMOF-20 and MOF-177 under similar pressure and temperature conditions (6.1, 6.2 and 7.0 mass%, respectively) [A. G. Wong-Foy et al. J. Am. Chem. Soc., 2006, 128, 3494; M. Dinca and J. R. Long. Angew. Chem., Int. Ed., 2008, 47, 6766]. Nevertheless the remarkable chemical and thermal stability of UiO-67 and the absence of Cr in its structure would make this material competitive.