Configuration interaction computational aspects

The Section on More Quantitive Aspects of Electronic Structure Calculations introduces many of the computational chemistry methods that are used to quantitatively evaluate molecular orbital and configuration mixing amplitudes. The Hartree-Fock self-consistent field (SCF), configuration interaction (Cl), multiconfigurational SCF (MCSCF), many-body and Mpller-Plesset perturbation theories,... 

The incorporation of electron correlation effects in a relativistic framework is considered. Three post Hartree-Fock methods are outlined after an introduction that defines the second quantized Dirac-Coulomb-Breit Hamiltonian in the no-pair approximation. Aspects that are considered are the approximations possible within the 4-component framework and the relation of these to other relativistic methods. The possibility of employing Kramers restricted algorithms in the Configuration Interaction and the Coupled Cluster methods are discussed to provide a link to non-relativistic methods and implementations thereof. It is shown how molecular symmetry can be used to make computations more efficient. 

It should be emphasized that basis set superposition effects exist in all theoretical and computational methods which involve the algebraic approximation, in matrix Hartree-Fock calculations, in configuration interaction and in many-body perturbation theory. It is frequently found that basis set superposition effects are larger in calculations which take account of electron correlation effects than they are in orbital theories such as the matrix Hartree-Fock method. We shall discuss this aspect further in Section VII.C. It should... 

The situation is even more complicated for explicitly correlated wave functions, like those from a configuration interaction (Cl) calculation. To extend the approach of Becke and Edgecombe to the correlated level, the Laplacian of spherically averaged correlated conditional same-spin pair density needs to be computed. Consistently, at the same time the calibration should be performed using the correlated uniform electron gas as well, an aspect that was not taken into account in any ELF analysis of correlated wave functions. Of course, the expression for fuUy correlated ELFbe would deviate from the original (HF) ELFbe formula. 

Of the 33 invited speakers and the seven who contributed talks, 17 accepted our invitation to contribute a chapter to this book. These chapters are complemented by three additional chapters from individuals to help develop a more cohesive book as well as an overview chapter. Approximately half of the chapters are focused on the development of ab initio electronic structure methods and consideration of specific challenging molecular systems using electronic structure theory. Some of these chapters document the dramatic developments in the range of applicability of the coupled-cluster method, including enhancements to coupled-cluster wavefunctions based on additional small multireference configuration interaction (MR-CISD) calculations, the method of moments, the similarity transformed equation of motion (STEOM) method, a state-specific multireference coupled-cluster method, and a computationally efficient approximation to variational coupled-cluster theory. The concentration on the coupled-cluster approach is balanced by an approximately equal number of chapters discussing other aspects of modem electronic stracture theory. In particular, other methods appropriate for the description of excited electronic states, such as multireference... 

A major drawback of the RFIM, however, is that it focuses entirely on the aspect of disorder, whereas confinement plays no role. To accormt for this problem, more recent theoretical studies, and computer simulations, of fluids in disordered media employ the concept of a quenched-annealed (QA) mixture [290, 291]. Here, the fluid molecules (the annealed species) equilibrate in a matrix consisting of particles quenched in a disordered (configuration. Thus, QA models combine both disorder and confinement, the latter being guaranteed by the finite size of the matrix particles. In addition, preferential adsorption can be realized by assiuning attractive (or other, more complex) interactions between fluid and matrix particles. 

Abstract The Vibrational Circular Dichroism (VCD) spectroscopy has been developing rapidly in both experimental and theoretical aspects. Currently, the VCD has become one of the most effective and reliable spectroscopic technique to determine the absolute configuration of chiral molecules. Its success is related to the availability of instrumentation and software for quantum-chemical calculation of the spectra. Nowadays, large parts of the VCD spectra can be trustfully predicted by theory and critically verified by confiding experiment, and vice versa. In the last decade, several theoretical and experimental VCD studies reported on VCD chirality transfer phenomenon occurring when an achiral molecule becomes VCD active as a result of intermolecular interactions with a chiral one. There are still some theoretical and experimental uncertainties about the VCD chirality transfer, however, benefits from an comprehensive use of the phenomenon can push our ability to diversify the intermolecular complexes and deepen our understanding of intermolecular interactions. This chapter is a review of the computational studies on VCD chirality transfer phenomenon supported by the experimental references, and ended by perspectives. 

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