XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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New coupled cluster methods for bond-breaking potential energy surfaces
Shuhua Li, Jun Shen, Enhua Xu, Zhuangfei Kou
School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of Ministry of Education, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing, 210093, P. R. China. shuhua@nju.edu.cn

In this talk, I will present our recent advances in coupled cluster methods for treating bond-breaking potential energy surfaces. The first approach is the coupled cluster singles, doubles with partial triples and quadruples based on the unrestricted Hartree-Fock (UHF) reference [1]. In this approach, canonical UHF molecular orbitals are first transformed into corresponding orbitals so that each -spin orbital is paired with only one -spin orbital. The method computationally scales as the seventh power of the system size. Test applications demonstrate that this method provides much more accurate descriptions for single-bond breaking processes than the UHF-based CCSD(T) method. Another approach is an approximate coupled cluster singles and doubles, with a hybrid treatment of triple excitations [denoted as CCSD(T)-h] [2,3]. With the concept of active and inactive corresponding orbitals (occupied or virtual), triple excitations can be divided into two subsets: (1) “active” triples involving at least one occupied active orbital and one virtual active orbital and (2) the remaining triples. The amplitudes of these two classes of triple excitations are obtained via two different approaches. The present method has been applied to study the bond-breaking potential energy surfaces in a number of small molecules. For all systems under study, the overall performance of CCSD(T)-h is very competitive with that of CCSDT, and much better than that of the UHF-based CCSD(T).

References:
[1] E. Xu, J. Shen, Z. Kou, S. Li J. Chem. Phys. 132, 134110 (2010).
[2] J. Shen, E. Xu, Z. Kou, S. Li J. Chem. Phys. 132, 114115 (2010).
[3] J. Shen, E. Xu, Z. Kou, S. Li J. Chem. Phys. revised, 2010.



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