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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 choice of the exponential ansatz is opportune because (unlike other ansatzes, for example, configuration interaction) it guarantees the size extensivity of the solution. Size consistency in CC theory, also unlike other theories, does not depend on the size consistency of the reference wave function.
Davidson correction improves both size consistency and size extensivity of CISD energies. [2] [4] Therefore, Davidson correction is frequently referred to in literature as size-consistency correction or size-extensivity correction. However, neither Davidson correction itself nor the corrected energies are size-consistent or size-extensive.
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 ...
An extensive property is a physical quantity whose value is proportional to the size of the system it describes, [8] or to the quantity of matter in the system. For example, the mass of a sample is an extensive quantity; it depends on the amount of substance.
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Sketch showing the interdependence of some multi-reference wavefunction methods, indicating the dependency on CASSCF of CASPTn method. Complete active space perturbation theory (CASPTn) is a multireference electron correlation method for computational investigation of molecular systems, especially for those with heavy atoms such as transition metals, lanthanides, and actinides.
For example, records for rainfall within an area might increase in three ways: records for additional time periods; records for additional sites with a fixed area; records for extra sites obtained by extending the size of the area. In such cases, the property of consistency may be limited to one or more of the possible ways a sample size can grow.
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