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Combination of approximate spin-projection and spin-restricted calculations based on ONIOM method for geometry optimization of large biradical systems

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Abstract

In this article, we combine the approximate spin projection (AP) and the spin-restricted (R) calculations based on ONIOM method for a geometry optimization of large biradical systems such as polynuclear metal complexes. This two-layer quantum mechanics/quantum mechanics′ (QM/QM′) (AP/R) method treats atoms of an inner layer around the spin sources with the AP method, whereas the whole atoms including the peripheral ligands are calculated by the R method. The AP/R method is applied for the geometry optimizations of chromium(II) ions in Cr2(O2CCH3)4(OH2)2 (1) and Cr2{O2CPh(CHMe2)3}4 (2) complexes that involve quadruple Cr[BOND]Cr bonds. The optimized structural parameters indicate that the error of the two-layer combination is smaller than 0.01 Å in the optimized Cr[BOND]Cr bond length on both complexes. The error is only 12.3% of the spin contamination error and 1.7% of the static correlation correction in the complex 1. The error of the two-layer combination in a total energy is also estimated to be lower than 1 kcal mol−1 in both complexes. © 2012 Wiley Periodicals, Inc.

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