A computational technique for solving the MP2 equations for periodic systems using a local-correlation approach and implemented in the CRYSCOR code is presented. The Hartree-Fock solution provided by the CRYSTAL program is used as a reference. The motivations for the implementation of the new code are discussed, and the techniques adopted are briefly recalled. With respect to the original formulation (Pisani et al., J Chem Phys 2005, 122, 094113), many new features have been introduced in CRYSCOR to improve its efficiency and robustness. In particular, an adaptation of the density fitting scheme to translationally periodic systems is described, based on Fourier transformation techniques. Three examples of application are provided, concerning the CO(2) crystal, proton transfer in ice XI, and the adsorption of methane on MgO (001). The results obtained with the periodic LMP2 method for these systems appear more reliable than the ones obtained using density functional theory.

The identification and quantification of defects is undoubtedly a thorough challenge in the characterization of "defect engineered" metal-organic frameworks (MOFs). UiO-66, known for its exceptional stability and defect tolerance, has been a popular target for defect engineering studies. Herein, we show that synthesizing UiO-66 in the presence of an excess of benzoic acid is a reliable method for obtaining UiO-66 samples with a very high concentration of missing cluster defects, allowing to modulate specific properties (i.e. surface area and hydrophobicity). This was elucidated by a multitechnique marriage of experimental and computational methods: a combination of PXRD, dissolution/ 1 H NMR spectroscopy, and N2 sorption measurements were used to quantify the defect loading, while vibrational spectroscopies (FTIR and Raman) allowed us to unequivocally identify the defect structure by comparison with DFT simulated spectra and visual analysis of the computed vibrational modes. to instead focus on isoreticularly expanded and/or functionalized UiO-66 derivatives (e.g. UiO-67, UiO-66-NH2) which offer improvements in these areas 16-33. However, these derivatives generally harbor two rather significant drawbacks with respect to UiO-66: lower stability 18,21,23,24,34,35 and higher cost. The bulk of their expense can be attributed to the functionalized and/or extended organic linker(s), many of which are not even commercially available, necessitating the development of in-house synthesis procedures 36 which can take months (or longer) to optimize unless the molecule has already been reported in the literature. It is therefore clear that an alternative approach for tuning UiO-66's porosity and functionality (and thus, performance) is desired. One such alternative approach is "defect engineering", a term often used as shorthand for the practice of manipulating defects via controlled, synthetic means 37-46. This approach has proven to be particularly effective for UiO-66 16,37,38,43-72 , whose structure is famed for its ability to tolerate an extraordinarily high concentration of defects while retaining much of its stability 37 (except for certain cases where the thermal stability has been shown to be heavily compromised 73 , and one sample (50Benz-HA in ref. [ 49 ]) which was found to have reduced water stability). We stress that this fascinating trait is not just an academic curiosity; many studies have shown

CRYSCOR is a periodic post-Hartree-Fock program based on local functions in direct space, i.e., Wannier functions and projected atomic orbitals. It uses atom centered Gaussians as basis functions. The Hartree-Fock reference, as well as symmetry information, is provided by the CRYSTAL program. CRYSCOR presently features an efficient and parallel implementation of periodic local second order Møller-Plesset perturbation theory (MP2), which allows us to study 1D-, 2D-and 3D-periodic systems beyond 1000 basis functions per unit cell. Apart from the correlation energy also the MP2 density matrix, and from that the Compton profile, are available. Very recently, a new module for calculating excitonic band gaps at the uncorrelated Configuration-Interaction-Singles (CIS) level has been added. Other advancements include new extrapolation techniques for calculating surface adsorption on semiinfinite solids. In this paper the diverse features and recent advances of the present CRYSCOR version are illustrated by exemplary applications to various systems: the adsorption of an argon monolayer on the MgO (100) surface, the rolling energy of a boron nitride nanoscroll, the relative stability of different aluminosilicates, the inclusion energy of methane in methane-ice-clathrates, and the effect of electron correlation on charge and momentum density of a-quartz. Furthermore, we present some first tentative CIS results for excitonic band gaps of simple 3D-crystals, and their dependence on the diffuseness of the basis set.

A comparative analysis of the intermolecular energy for a data set including 60 molecular crystals with a large variety of functional groups has been carried out using three different computational approaches: (i) a method based on a physically meaningful empirical partition of the interaction energy (PIXEL), (ii) density functional methods with a posteriori empirical correction for the dispersion interactions (DFT-D), and (iii) a full periodic ab initio quantum mechanical method based on Møller-Plesset perturbation theory for the electron correlation using localized crystal orbitals (LMP2). Due to the large computational cost, LMP2 calculations have been restricted to a subset of seven molecular crystal comprising benzene, formic acid, formamide, succinic anhydride, urea, oxalic acid, and nitroguanidine, and the results compared with PIXEL and DFT-D data as well as with the experimental data show excellent agreement among all adopted methods. This shows that both DFT-D and PIXEL approaches are robust predictive tools for studying molecular crystals. A detailed analysis shows a very similar dispersion contribution of the two methods across the 60 considered molecular crystals. The study also confirms that pure DFT shows serious deficiencies in properly handling molecular crystals in which the dispersive contribution is large. Due to the negligible requested computational resources, PIXEL is the method of choice in screening of a large number of molecular crystals, an essential step to predict crystal polymorphism or to study crystal growth processes. DFT-D can then be used to refine the ranking emerged from PIXEL calculations due to its general applicability and robustness in properly handling short-range interactions.