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This study focuses on the selection of the most accurate and computationally feasible correlated ab initio methods and basis sets in theoretical chemistry. The goal is to ensure size consistency in calculations, which is crucial for achieving reliable energy values for molecular systems. Size consistency implies that the energy calculated for two atoms or fragments separated by a significant distance should equal the sum of their individual energies. We analyze how size-consistent and non-size-consistent methods yield different results and their implications for bond energy calculations in molecules like N2.
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Model chemistry: theoretical method and basis set Goal: select the most accurate calculation that is computationally feasible for a given molecular system
size-consistent – a calculation that gives the same energy for two atoms (or molecular fragments) separated by a large distance as is obtained from summing the energies for the atoms (or molecular fragments) computed separately. So for a size-consistent method, the bond energy in N2 is ΔEdiss = 2E(N) - E(N2). For a method that is not size-consistent, a "supermolecule" calculation with a big distance (e.g., 100 Å) is required: ΔEdiss = E(N......N) - E(N2).
