A unitary group based open‐shell coupled cluster study of vibrational frequencies in ground and excited states of first row diatomics

1996 ◽  
Vol 104 (23) ◽  
pp. 9555-9562 ◽  
Author(s):  
Xiangzhu Li ◽  
Josef Paldus
Keyword(s):  
Excited States ◽  
Unitary Group ◽  
Open Shell ◽  
Vibrational Frequencies ◽  
Coupled Cluster

Coupled-cluster calculations for ground and excited states of closed- and open-shell nuclei using methods of quantum chemistry

2005 ◽  
Vol 31 (8) ◽  
pp. S1291-S1299 ◽  
Author(s):  
Marta Włoch ◽  
Jeffrey R Gour ◽  
Piotr Piecuch ◽  
David J Dean ◽  
Morten Hjorth-Jensen ◽  
...  
Keyword(s):  
Quantum Chemistry ◽  
Excited States ◽  
Open Shell ◽  
Coupled Cluster ◽  
Cluster Calculations ◽  
Coupled Cluster Calculations

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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