Density functional theory transition metal compound structure

For quantum chemistry, first-row transition metal complexes are perhaps the most difficult systems to treat. First, complex open-shell states and spin couplings are much more difficult to deal with than closed-shell main group compounds. Second, the Hartree—Fock method, which underlies all accurate treatments in wavefunction-based theories, is a very poor starting point and is plagued by multiple instabilities that all represent different chemical resonance structures. On the other hand, density functional theory (DFT) often provides reasonably good structures and energies at an affordable computational cost. Properties, in particular magnetic properties, derived from DFT are often of somewhat more limited accuracy but are still useful for the interpretation of experimental data. 

Before any computational study on molecular properties can be carried out, a molecular model needs to be established. It can be based on an appropriate crystal structure or derived using any technique that can produce a valid model for a given compound, whether or not it has been prepared. Molecular mechanics is one such technique and, primarily for reasons of computational simplicity and efficiency, it is one of the most widely used technique. Quantum-mechanical modeling is far more computationally intensive and until recently has been used only rarely for metal complexes. However, the development of effective-core potentials (ECP) and density-functional-theory methods (DFT) has made the use of quantum mechanics a practical alternative. This is particularly so when the electronic structures of a small number of compounds or isomers are required or when transition states or excited states, which are not usually available in molecular mechanics, are to be investigated. However, molecular mechanics is still orders of magnitude faster than ab-initio quantum mechanics and therefore, when large numbers of... 

The structure of the intermediate implicitly encompasses molecular, electronic, and vibrational components where the molecular structure is most commonly deduced by X-ray crystallography. More limited structural data may also be obtained from solute species through analysis of the X-ray absorption fine structure (XAFS) spectra and this will be discussed briefly in Section 1.6. Clearly the electronic and vibrational structure must be obtained from analysis of the spectra. The interconnection between these aspects of the structure is reinforced by in-silico techniques, where advances in DFT (density-functional theory) have greatly expanded the range of transition-metal compounds and... 

The Amsterdam Density Functional package (ADF) is software for first-principles electronic structure calculations (quantum chemistry). ADF is often used in the research areas of catalysis, inorganic and heavy-element chemistry, biochemistry, and various types of spectroscopy. ADF is based on density functional theory (DFT) (see Chapter 2.39), which has dominated quantum chemistry applications since the early 1990s. DFT gives superior accuracy to Hartree-Fock theory and semi-empirical approaches, especially for transition-metal compounds. In contrast to conventional correlated post-Hartree-Fock methods, it enables accurate treatment of systems with several hundreds of atoms (or several thousands with QM/MM)." ... 

Electron structure calculations often become difficult when transition metals are involved. If the system has incomplete d shells many electron configurations contribute even to the ground state leading to non-dynamical electron correlation. Wave function-based methods with multideterminant references are required for high accuracy. Density functional theory is often successful, but no current functional is reliable for transition metals compounds in general. QMC as a wave function-based method has to use multideterminant... 

Density Functional Applications Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field NMR Chemical Shift Computation Ab Initio NMR Data Correlation with Chemical Structure NMR of Transition Metal Compounds. 

We start with the question of what happens to the large orbital moment of f electrons when they are hybridized with other states in solids. This question, of course, is central to understanding the unusual properties of actinide (and cerium) compounds. Form-factor measurements had shown the importance of hybridization effects in compounds such as UGej (Lander et al. 1980), but at that time no theory had been developed to handle these effects in particular the orbital contribution was known to be incorrectly treated in band-structure calculations (Brooks et al. 1984, Brooks 1985). Brooks, Johansson, and their collaborators corrected this deficiency by adding an orbital polarization term in the density-functional approximation (see the chapter by Brooks and Johansson (ch. 112) in this volume). When they made calculations on a series of intermetallic compounds, particularly those with a transition metal in the compact fee Laves phase, they found that the value of was reduced compared to the free-ion values. Loosely speaking, we can associate such a partial quenching of the /j ,-value with the fact that the 5f electrons have become partially itinerant, and we know that fully itinerant electrons (in the 3d metals, for example) have 0. 

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