MC-PDFT Can Calculate Singlet-Triplet Splittings of Organic Diradicals.

The singlet-triplet splittings of a set of diradical organic molecules are calculated using multiconfiguration pair-density functional theory (MC-PDFT) and the results are compared with those obtained by Kohn-Sham density functional theory (KS-DFT) and complete active space second-order perturbation theory (CASPT2) calculations. We found that MC-PDFT, even with small and systematically defined active spaces, is competitive in accuracy with CASPT2, and it yields results with greater accuracy and precision than Kohn-Sham DFT with the same parent functional. MC-PDFT also avoids the challenges associated with spin contamination in KS-DFT. It is also shown that MC-PDFT is much less computationally expensive than CASPT2 when applied to larger active spaces, and this illustrates the promise of this method for larger diradical organic systems

Beyond Density Functional Theory: the Multiconfigurational Approach to Model Heterogeneous Catalysis

Catalytic processes are crucially important for many practical chemical applications. Heterogeneous catalysts are especially appealing because of their high stability and the relative ease with which they may be recovered and reused. Computational modeling can play an important role in the design of more catalytically active materials through the identification of reaction mechanisms and the opportunity to assess hypothetical catalysts in silico prior to experimental verification. Kohn-Sham density functional theory (KS-DFT) is the most used method in computational catalysis because it is affordable and it gives results of reasonable accuracy in many instances. Furthermore, it can be employed in a “black-box” mode that does not require significant a priori knowledge of the system. However, KS-DFT has some limitations: it suffers from self-interaction error (sometime referred to as delocalization error), but a greater concern is that it provides an intrinsically single-reference description of the electronic structure, and this can be especially problematic for modeling catalysis when transition metals are involved. In this perspective, we highlight some noteworthy applications of KS-DFT to heterogeneous computational catalysis, as well as cases where KS-DFT fails accurately to describe electronic structures and intermediate spin states in open-shell transition metal systems. We next provide an introduction to state-of-the-art multiconfigurational (MC; also referred to as multireference (MR)) methods and their advantages and limitations for modeling heterogeneous catalysis. We focus on specific examples to which MC methods have 2 been applied and discuss the challenges associated with these calculations. We conclude by offering our vision for how the community can make further progress in the development of MC methods for application to heterogeneous catalysis

Full Correlation in a Multiconfigurational Study of Bimetallic Clusters : Restricted Active Space Pair-Density Functional Theory Study of [2Fe-2S] Systems

Iron-sulfur clusters play a variety of important roles in protein chemistry, and understanding the energetics of their spin ladders is an important part of understanding these roles. Computational modeling can offer considerable insight into such problems; however, calculations performed thus far on systems with multiple transition metals have typically either been restricted to a single-configuration representation of the density, as in Kohn-Sham theory, or been limited to correlating excitations only within an active space, as in active-space self-consistent field methods. For greater reliability, a calculation should include full correlation, i.e., not only correlation internal to the active space but also external correlation, and it is desirable to combine this full electron correlation with a multiconfigurational representation of the wave function; but this has been impractical thus far. Here we present an affordable way to do that by using restricted-active-space pair-density functional theory. We show that with this method it is possible to compute the entire spin ladder for systems containing two Fe centers bridged by two S atoms. On the other hand, with second-order perturbation theory only the high-spin states can be computed. A key result is that, in agreement with some experiments, we find a high-spin ground state for a relaxed reduced [Fe2S2(SCH3)4] 3- cluster, which is a novel result in computational studies

Metal-Organic Frameworks with Metal Catecholates for O2/N2 Separation

Oxygen and nitrogen are widely produced feedstocks with diverse fields of applications, but are primarily obtained via the energy-intensive cryogenic distillation of air. More energy-efficient processes are desirable, and materials such as zeolites and metal-organic frameworks (MOFs) have been studied for air separation. Inspired by recent theoretical work identifying metal-catecholates for enhancement of O2 selectivity MOFs, in this work the computation-ready experimental (CoRE) database of MOF structures was screened to identify promising candidates for incorporation of metal catecholates. Based on structural requirements, preliminary Grand-Canonical Monte Carlo simulations, and further constraints to ensure the computational feasibility, over 5,000 structures were eliminated and four MOFs (UiO-66(Zr), Ce-UiO-66, MOF-5, and IRMOF-14) were treated with periodic density functional theory (DFT). Metal catecholates (Mg, Co, Ni, Zn, and Cd) were selected based on cluster DFT calculations and were added to the shortlisted MOFs. Periodic DFT was used to compute O2 and N2 binding energies near metal catecholates. We find that the binding energies are primarily dependent on the metals in the metal catecholates, all of which bind O2 quite strongly (80-258 kJ/mol) and have weaker binding for N2 (3-148 kJ/mol). Of those studied here, Cd-catecholated MOFs are identified as the most promisin

Origin of the strong interaction between polar molecules and copper(II) paddle-wheels in metal organic frameworks

The copper paddle-wheel is the building unit of many metal organic frameworks. Because of the ability of the copper cations to attract polar molecules, copper paddle-wheels are promising for carbon dioxide adsorption and separation. They have therefore been studied extensively, both experimentally and computationally. In this work we investigate the copper–CO2 interaction in HKUST-1 and in two different cluster models of HKUST-1: monocopper Cu(formate)2 and dicopper Cu2(formate)4. We show that density functional theory methods severely underestimate the interaction energy between copper paddle-wheels and CO2, even including corrections for the dispersion forces. In contrast, a multireference wave function followed by perturbation theory to second order using the CASPT2 method correctly describes this interaction. The restricted open-shell Møller–Plesset 2 method (ROS-MP2, equivalent to (2,2) CASPT2) was also found to be adequate in describing the system and used to develop a novel force field. Our parametrization is able to predict the experimental CO2 adsorption isotherms in HKUST-1, and it is shown to be transferable to other copper paddle-wheel systems

Civil society leadership in the struggle for AIDS treatment in South Africa and Uganda

Includes abstract.Includes bibliographical references.This thesis is an attempt to theorise and operationalise empirically the notion of ‘civil society leadership’ in Sub-Saharan Africa. ‘AIDS leadership,’ which is associated with the intergovernmental institutions charged with coordinating the global response to HIV/AIDS, is both under-theorised and highly context-specific. In this study I therefore opt for an inclusive framework that draws on a range of approaches, including the literature on ‘leadership’, institutions, social movements and the ‘network’ perspective on civil society mobilisation. This framework is employed in rich and detailed empirical descriptions (‘thick description’) of civil society mobilisation around AIDS, including contentious AIDS activism, in the key case studies of South Africa and Uganda. South Africa and Uganda are widely considered key examples of poor and good leadership (from national political leaders) respectively, while the Treatment Action Campaign (TAC) and The AIDS Support Organisation (TASO) are both seen as highly effective civil society movements. These descriptions emphasise ‘transnational networks of influence’ in which civil society leaders participated (and at times actively constructed) in order to mobilise both symbolic and material resources aimed at exerting influence at the transnational, national and local levels

Air Separation by Catechol-Ligated Transition Metals: A Quantum Chemical Screening

The separation of O₂ and N₂ from air is of great importance in a variety of industrial contexts, but the primary means of accomplishing the separation is cryogenic distillation, an energy-intensive process. A material that could enable air separation to occur at conventional temperatures would be of great economic and environmental benefit. Metalated catecholates within metal–organic frameworks have been considered for other gas separations and are shown here to have significant potential for air separation. Calculations of interaction energies between catecholates with first-row transition metals and guests O₂ and N₂ were performed using density functional theory and multireference complete active space self-consistent field followed by second-order perturbation theory. A general recipe is offered for active space selection for metalated catecholate systems. The multireference results are used to rationalize O₂ binding in terms of redox activity with the metalated catecholate. O₂ is predicted to bind more strongly than N₂ for all cases except Cu²⁺, with general agreement in the binding trends among all methods

Automation of Active Space Selection for Multireference Methods via Machine Learning on Chemical Bond Dissociation

Predicting and understanding the chemical bond is one of the major challenges of computational quantum chemistry. Kohn−Sham density functional theory (KS-DFT) is the most common method, but approximate density functionals may not be able to describe systems where multiple electronic configurations are equally important. Multiconfigurational wave functions, on the other hand, can provide a detailed understanding of the electronic structure and chemical bond of such systems. In the complete-active-space self-consistent field (CASSCF) method one performs a full configuration interaction calculation in an active space consisting of active electrons and active orbitals. However, CASSCF and its variants require the selection of these active spaces. This choice is not black-box; it requires significant experience and testing by the user, and thus active space methods are not considered particularly user-friendly and are employed only by a minority of quantum chemists. Our goal is to popularize these methods by making it easier to make good active space choices. We present a machine learning protocol that performs an automated selection of active spaces for chemical bond dissociation calculations of main group diatomic molecules. The protocol shows high prediction performance for a given target system as long as a properly correlated system is chosen for training. Good active spaces are correctly predicted with a considerably better success rate than random guess (larger than 80% precision for most systems studied). Our automated machine learning protocol shows that a “black-box” mode is possible for facilitating and accelerating the large-scale calculations on multireference systems where single-reference methods such as KS-DFT cannot be applied.</div

Systematic Expansion of Active Spaces beyond the CASSCF Limit: A GASSCF/SplitGAS Benchmark Study

The applicability and accuracy of the generalized active space self-consistent field, (GASSCF), and (SplitGAS) methods are presented. The GASSCF method enables the exploration of larger active spaces than with the conventional complete active space SCF, (CASSCF), by fragmentation of a large space into subspaces and by controlling the interspace excitations. In the SplitGAS method, the GAS configuration interaction, CI, expansion is further partitioned in two parts: the principal, which includes the most important configuration state functions, and an extended, containing less relevant but not negligible ones. An effective Hamiltonian is then generated, with the extended part acting as a perturbation to the principal space. Excitation energies of ozone, furan, pyrrole, nickel dioxide, and copper tetrachloride dianion are reported. Various partitioning schemes of the GASSCF and SplitGAS CI expansions are considered and compared with the complete active space followed by second-order perturbation theory, (CASPT2), and multireference CI method, (MRCI), or available experimental data. General guidelines for the optimum applicability of these methods are discussed together with their current limitations