Real or artifactual symmetry breaking in the BNB radical: A multireference coupled cluster viewpoint

2007 ◽  
Vol 126 (22) ◽  
pp. 224304 ◽  
Author(s):  
Xiangzhu Li ◽  
Josef Paldus
Keyword(s):  
Symmetry Breaking ◽  
Coupled Cluster

Energies and analytic gradients for a coupled-cluster doubles model using variational Brueckner orbitals: Application to symmetry breaking in O4+

1998 ◽  
Vol 109 (11) ◽  
pp. 4171-4181 ◽  
Author(s):  
C. David Sherrill ◽  
Anna I. Krylov ◽  
Edward F. C. Byrd ◽  
Martin Head-Gordon
Keyword(s):  
Symmetry Breaking ◽  
Coupled Cluster ◽  
Brueckner Orbitals

scholarly journals Energy and Analytic Gradients for the Orbital-Optimized Coupled-Cluster Doubles Method with the Density-Fitting Approximation: An Efficient Implementation

2020 ◽  
Author(s):  
Uğur Bozkaya ◽  
Aslı Ünal ◽  
Yavuz Alagöz
Keyword(s):  
Symmetry Breaking ◽  
Hydrogen Transfer ◽  
Computational Cost ◽  
Linear Response Theory ◽  
Open Shell ◽  
Vibrational Frequencies ◽  
Basis Set ◽  
Computational Time ◽  
Coupled Cluster ◽  
Density Fitting

Efficient implementations of the orbital-optimized coupled-cluster doubles [or simply ``optimized CCD'', OCCD, for short] method and its analytic energy gradients with the density-fitting (DF) approach, denoted by DF-OCCD, are presented. In addition to the DF approach, the Cholesky-decomposed variant (CD-OCCD) is also implemented for energy computations. The computational cost of the DF-OCCD method {(available in a plugin version of the {\sc DFOCC} module of {\sc Psi4})} is compared with that of the conventional OCCD {(from the {\sc Q-Chem} package)}. The OCCD computations were performed with the {\sc Q-chem} package, in which it is denoted by OD. In the conventional OCCD, one needs to perform four-index integrals transformations at each CCD iterations, which limits its applications to large chemical systems. Our results demonstrate that DF-OCCD provides dramatically lower computational costs compared to OCCD, there are almost 8-fold reductions in the computational time for the \ce{C6H14} molecule with the cc-pVTZ basis set. For open-shell geometries, interaction energies, and hydrogen transfer reactions, DF-OCCD provides significant improvements upon DF-CCD. {Further, the performance of the DF-OCCD method is substantially better for harmonic vibrational frequencies in the case of symmetry breaking problems. Moreover,} several factors make DF-OCCD more attractive compared to CCSD: (1) for DF-OCCD there is no need for orbital relaxation contributions in analytic gradient computations (2) active spaces can readily be incorporated into DF-OCCD (3) DF-OCCD provides accurate vibrational frequencies when symmetry-breaking problems are observed (4) in its response function, DF-OCCD avoids artificial poles; hence, excited-state molecular properties can be computed via linear response theory (5) Symmetric and asymmetric triples corrections based on DF-OCCD [DF-OCCD(T)] has a significantly better performance in near degeneracy regions.

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