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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Davidson correction ： ウィキペディア英語版 Davidson correction The Davidson correction is an energy correction often applied in calculations using the method of truncated configuration interaction, which is one of several post-Hartree–Fock ab initio quantum chemistry methods in the field of computational chemistry. It was introduced by Ernest R. Davidson. It allows one to estimate the value of the full configuration interaction energy from a limited configuration interaction expansion result, although more precisely it estimates the energy of configuration interaction up to quadruple excitations (CISDTQ) from the energy of configuration interaction up to double excitations (CISD). It uses the formula :$\backslash Delta\; E\_Q\; =\; (1\; -\; a\_0^2)(E\_\; -\; E\_),\; \backslash$ :$E\_\; \backslash approx\; E\_\; +\; \backslash Delta\; E\_Q,\; \backslash$ where ''a''0 is the coefficient of the Hartree–Fock wavefunction in the CISD expansion, ''E''CISD and ''E''HF are the energies of the CISD and Hartree–Fock wavefunctions respectively, and Δ''EQ'' is the correction to estimate ''E''CISDTQ, the energy of the CISDTQ wavefunction. Such estimation is based on perturbation theory analysis. Therefore, CISD calculations with the Davidson correction included are frequently referred to as CISD(Q). == Application == Davidson correction is very popular due to its low computational cost. The correction improves contribution of electron correlation to the energy. 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. Davidson correction does not give information about wave function. Therefore it cannot be used to correct wave-function-dependent quantities such as dipole moment, charge density and vibronic couplings. Analytical gradients for Davidson corrections are in general not available in quantum chemistry programs. As with other perturbative approaches, Davidson correction is not reliable when the electronic structure of CISD and the reference Hartree–Fock wave functions are significantly different. The correction is invalid, when $a\_0^2$ is not close to 1. This happens when multi-reference character is significant or when CISD is used to calculate a state that is not the reference state, for example, an excited state or a state with different spin multiplicity. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Davidson correction」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.