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In quantum chemistry, size consistency and size extensivity are concepts relating to how the behaviour of quantum-chemistry calculations changes with the system size. Size consistency (or strict separability) is a property that guarantees the consistency of the energy behaviour when interaction between the involved molecular subsystems is nullified (for example, by distance).
The size-consistency and size-extensivity problems of truncated CI are alleviated but still exist. In small molecules, accuracy of the corrected energies can be similar to results from coupled cluster theory calculations. The Davidson correction does not give information about the wave function.
The resulting equations are a set of non-linear equations, which are solved in an iterative manner. Standard quantum-chemistry packages (GAMESS (US), NWChem, ACES II, etc.) solve the coupled-cluster equations using the Jacobi method and direct inversion of the iterative subspace extrapolation of the t-amplitudes to accelerate convergence.
According to International Union of Pure and Applied Chemistry (IUPAC), an intensive property or intensive quantity is one whose magnitude is independent of the size of the system. [3] An intensive property is not necessarily homogeneously distributed in space; it can vary from place to place in a body of matter and radiation.
Quadratic configuration interaction [1] (QCI) is an extension of configuration interaction [2] that corrects for size-consistency errors in single and double excitation CI methods (CISD). [ 3 ] Size-consistency means that the energy of two non-interacting (i.e. at large distance apart) molecules calculated directly will be the sum of the ...
Semi-empirical quantum chemistry methods Møller–Plesset perturbation theory Configuration interaction Coupled cluster Multi-configurational self-consistent field Quantum chemistry composite methods Quantum Monte Carlo: Density functional theory; Time-dependent density functional theory Thomas–Fermi model Orbital-free density functional theory
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In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform.