Complete spin contamination-free MP2 method

[Display omitted] •New methods to improve the UMP2, designated the UGA-UMP2 and UGA-SUMP2, are proposed.•Spin adaptation of the double excitation space for UMP2 is achieved based on the UGA.•Spin contamination present in UMP2 calculations is completely eliminated.•Several numerical results for the E...

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Bibliographic Details
Published in:Chemical physics letters 2017-02, Vol.669, p.224-229
Main Authors: Nishida, Manami, Lim, Fang-Han, Hori, Yuta, Ida, Tomonori, Mizuno, Motohiro
Format: Article
Language:English
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Summary:[Display omitted] •New methods to improve the UMP2, designated the UGA-UMP2 and UGA-SUMP2, are proposed.•Spin adaptation of the double excitation space for UMP2 is achieved based on the UGA.•Spin contamination present in UMP2 calculations is completely eliminated.•Several numerical results for the Ea show the good performance of the new methods. To improve the unrestricted second-order Møller-Plesset (UMP2) method, we propose a method in which spin contamination is eliminated due to spin adaptation of the double excitation space based on the unitary group approach (UGA), because the UMP2 calculations intrinsically involve spin contamination from the double excitation space. Numerical results for the activation energies of several radical reactions show that this UGA-UMP2 method offers substantial improvement over the UMP2 method. Furthermore, the UGA-SUMP2 method, which completely eliminates the spin contamination from both the reference and excitation space present in UMP2 calculations, is also formulated.
ISSN:0009-2614
1873-4448
DOI:10.1016/j.cplett.2016.12.039