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Lecture# Size Extensivity and Consistency in Quantum Chemistry

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This lecture covers the concepts of size extensivity and consistency in quantum chemistry, explaining how properties scale with system size and remain constant. It also delves into post-Hartree-Fock methods like Configuration Interaction, Many-Body Perturbation Theory, and Coupled Cluster Theory. The slides detail the expansion of perturbed wavefunctions in doubly excited Slater determinants and the correction to energy levels, showcasing the importance of electron correlation in improving results.

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Related lectures (13)

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Computational chemistry is a branch of chemistry that uses computer simulation to assist in solving chemical problems. It uses methods of theoretical chemistry, incorporated into computer programs, to calculate the structures and properties of molecules, groups of molecules, and solids. It is essential because, apart from relatively recent results concerning the hydrogen molecular ion (dihydrogen cation, see references therein for more details), the quantum many-body problem cannot be solved analytically, much less in closed form.

Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions to physical and chemical properties of molecules, materials, and solutions at the atomic level. These calculations include systematically applied approximations intended to make calculations computationally feasible while still capturing as much information about important contributions to the computed wave functions as well as to observable properties such as structures, spectra, and thermodynamic properties.

Configuration interaction (CI) is a post-Hartree–Fock linear variational method for solving the nonrelativistic Schrödinger equation within the Born–Oppenheimer approximation for a quantum chemical multi-electron system. Mathematically, configuration simply describes the linear combination of Slater determinants used for the wave function. In terms of a specification of orbital occupation (for instance, (1s)2(2s)2(2p)1...), interaction means the mixing (interaction) of different electronic configurations (states).

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Covers the Hartree-Fock Roothaan Equations and different types of Hartree-Fock methods, discussing their performance.

Covers the Slater-Condon rules for configuration interaction and determinants.

Delves into Moller-Plesset Perturbation Theory, discussing energy corrections and wavefunction expansion in quantum chemistry.