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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 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.
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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.
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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Size (the number of people involved) is an important characteristic of the groups, organizations, and communities in which social behavior occurs. [1] When only a few persons are interacting, adding just one more individual may make a big difference in how they relate.