The Experts below are selected from a list of 44019 Experts worldwide ranked by ideXlab platform
Rodney J Bartlett - One of the best experts on this subject based on the ideXlab platform.
-
perturbation improved natural linear scaled Coupled Cluster method and its application to conformational analysis
Journal of Physical Chemistry A, 2019Co-Authors: Rodney J BartlettAbstract:: The fragment-based Coupled-Cluster (CC) theory utilizing the transferable functional groups through natural localized molecular orbital (NLMO), that is, the natural linear-scaled Coupled-Cluster (NLSCC) has been further developed to take the extra-fragment interactions into account. The correction to the interaction energies sacrificed during the fragmentation process for the previous NLSCC method is computed by a computationally efficient perturbation theory that maintains the original scaling. The new linear-scaled Coupled-Cluster for the singles and doubles (CCSD) method is applied to the analysis of relative energies of delicate conformational problems of polypeptides. By adding a perturbation correction, results accurate to less than a kcal/mol are obtained for the alanine tetramer.
-
transition metal atomic multiplet states through the lens of single reference Coupled Cluster and the equation of motion Coupled Cluster methods
Theoretical Chemistry Accounts, 2014Co-Authors: Varun Rishi, Ajith Perera, Rodney J BartlettAbstract:Transition metal atoms require a correct description of quasi-degeneracy and spin states, which lead to many closely lying multiplets. Before embarking on the theoretical description of transition metal complexes, the problems encountered in the atoms should be shown to be amenable to a chosen ab initio electronic structure method. It is usually thought that multi-reference methods should be a necessity for the correct description of transition metal multiplets. To the contrary, this paper explores the use of single-reference Coupled-Cluster theory in most of its manifestations. These include a variety of orbital choices from variationally optimal restricted open-shell references, to Brueckner orbital references, fractionally occupied references, quasi-restricted Hartree–Fock and other choices. In addition, the equation-of-motion Coupled-Cluster method for target multiplet states is considered. Relativistic corrections are obtained from a Douglas–Kroll–Hess fifth-order approximation which is found to be superior to effective core potentials, although there is little coupling between the relativistic effects and electron correlation.
-
Coupled Cluster theory and its equation of motion extensions
Wiley Interdisciplinary Reviews: Computational Molecular Science, 2012Co-Authors: Rodney J BartlettAbstract:Coupled-Cluster theory offers today's reference quantum chemical method for most of the problems encountered in electronic structure theory. It has been instrumental in establishing the now well-known paradigm of converging, many-body methods, Many-body perturbation theory (MBPT) for second, MBPT2, and fourth-order MBPT4; and Coupled-Cluster (CC) theory for different categories of excitations, singles, doubles, triples, quadruples (SDTQ). Although built on the same basic concept as configuration interaction (CI), many-body methods fundamentally improve upon CI approximations by introducing the property of size extensivity, meaning that contrary to any truncated CI all terms properly scale with the number of electrons in the problem. This fundamental aspect of many-electron methods leads to the exceptional performance of CC theory and its finite-order MBPT approximations plus its equation-of-motion extensions for excited, ionized, and electron attached states. This brief overview will describe formal aspects of the theory which should be understood by perspective users of CC methods. We will also comment on some current developments that are improving the theory's accuracy or applicability. © 2011 John Wiley & Sons, Ltd.
-
rethinking linearized Coupled Cluster theory
Journal of Chemical Physics, 2009Co-Authors: Andrew G Taube, Rodney J BartlettAbstract:Hermitian linearized Coupled-Cluster methods have several advantages over more conventional Coupled-Cluster methods including facile analytical gradients for searching a potential energy surface. A persistent failure of linearized methods, however, is the presence of singularities on the potential energy surface. A simple Tikhonov regularization procedure is introduced that can eliminate this singularity. Application of the regularized linearized Coupled-Cluster singles and doubles (CCSD) method to both equilibrium structures and transition states shows that it is competitive with or better than conventional CCSD, and is more amenable to parallelization.
-
frozen natural orbital Coupled Cluster theory forces and application to decomposition of nitroethane
Journal of Chemical Physics, 2008Co-Authors: Andrew G Taube, Rodney J BartlettAbstract:The frozen natural orbital (FNO) Coupled-Cluster method increases the speed of Coupled-Cluster (CC) calculations by an order of magnitude with no consequential error along a potential energy surface. This method allows the virtual space of a correlated calculation to be reduced by about half, significantly reducing the time spent performing the Coupled-Cluster (CC) calculation. This paper reports the derivation and implementation of analytical gradients for FNO-CC, including all orbital relaxation for both noncanonical and semicanonical perturbed orbitals. These derivatives introduce several new orbital relaxation contributions to the CC density matrices. FNO-CCSD(T) and FNO-ΛCCSD(T) are applied to a test set of equilibrium structures, verifying that these methods are capable of reproducing geometries and vibrational frequencies accurately, as well as energies. Several decomposition pathways of nitroethane are investigated using CCSD(T) and ΛCCSD(T) with 60% of the FNO virtual orbitals in a cc-pVTZ basis,...
Frank Neese - One of the best experts on this subject based on the ideXlab platform.
-
a perturbative approach to multireference equation of motion Coupled Cluster
Molecular Physics, 2021Co-Authors: Marvin H Lechner, Robert Izsak, Marcel Nooijen, Frank NeeseAbstract:We introduce a variant of the multireference equation-of-motion Coupled-Cluster (MR-EOMCC) method where the amplitudes used for the similarity transformations are estimated from perturbation theory...
-
fragment based local Coupled Cluster embedding approach for the quantification and analysis of noncovalent interactions exploring the many body expansion of the local Coupled Cluster energy
Journal of Chemical Theory and Computation, 2021Co-Authors: Soumen Ghosh, Frank Neese, Robert Izsak, Giovanni BistoniAbstract:Herein, we introduce a fragment-based local Coupled Cluster embedding approach for the accurate quantification and analysis of noncovalent interactions in molecular aggregates. Our scheme combines two different expansions of the domain-based local pair natural orbital Coupled Cluster (DLPNO-CCSD(T)) energy: the many-body expansion (MBE) and the local energy decomposition (LED). The low-order terms in the MBE are initially computed in the presence of an environment that is treated at a low level of theory. Then, LED is used to decompose the energy of each term in the embedded MBE into additive fragment and fragment-pairwise contributions. This information is used to quantify the total energy of the system while providing at the same time in-depth insights into the nature and cooperativity of noncovalent interactions. Two different approaches are introduced and tested, in which the environment is treated at different levels of theory: the local Coupled Cluster in the Hartree-Fock (LCC-in-HF) method, in which the environment is treated at the HF level; and the electrostatically embedded local Coupled Cluster method (LCC-in-EE), in which the environment is replaced by point charges. Both schemes are designed to preserve as much as possible the accuracy of the parent local Coupled Cluster method for total energies, while being embarrassingly parallel and less memory intensive. These schemes appear to be particularly promising for the study of large and complex molecular aggregates at the Coupled Cluster level, such as condensed phase systems and protein-ligand interactions.
-
is it possible to obtain Coupled Cluster quality energies at near density functional theory cost domain based local pair natural orbital Coupled Cluster vs modern density functional theory
Journal of Chemical Theory and Computation, 2015Co-Authors: Dimitrios G Liakos, Frank NeeseAbstract:The recently developed domain-based local pair natural orbital Coupled Cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) delivers results that are closely approaching those of the parent canonical Coupled Cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2–3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2–4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality. The computational effort for the Coupled Cluster step scales nearly linearly with system size. Since, in many instances, the Coupled Cluster step in DLPNO-CCSD...
-
is it possible to obtain Coupled Cluster quality energies at near density functional theory cost domain based local pair natural orbital Coupled Cluster vs modern density functional theory
Journal of Chemical Theory and Computation, 2015Co-Authors: Dimitrios G Liakos, Frank NeeseAbstract:The recently developed domain-based local pair natural orbital Coupled Cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) delivers results that are closely approaching those of the parent canonical Coupled Cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2-3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2-4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality. The computational effort for the Coupled Cluster step scales nearly linearly with system size. Since, in many instances, the Coupled Cluster step in DLPNO-CCSD(T) is cheaper or at least not much more expensive than the preceding Hartree-Fock calculation, it is useful to compare the method against modern density functional theory (DFT), which requires an effort comparable to that of Hartree-Fock theory (at least if Hartree-Fock exchange is part of the functional definition). Double hybrid density functionals (DHDF's) even require a MP2-like step. The purpose of this article is to evaluate the cost vs accuracy ratio of DLPNO-CCSD(T) against modern DFT (including the PBE, B3LYP, M06-2X, B2PLYP, and B2GP-PLYP functionals and, where applicable, their van der Waals corrected counterparts). To eliminate any possible bias in favor of DLPNO-CCSD(T), we have chosen established benchmark sets that were specifically proposed for evaluating DFT functionals. It is demonstrated that DLPNO-CCSD(T) with any of the three default thresholds is more accurate than any of the DFT functionals. Furthermore, using the aug-cc-pVTZ basis set and the LoosePNO default settings, DLPNO-CCSD(T) is only about 1.2 times slower than B3LYP. With NormalPNO thresholds, DLPNO-CCSD(T) is about a factor of 2 slower than B3LYP and shows a mean absolute deviation of less than 1 kcal/mol to the reference values for the four different data sets used. Our conclusion is that Coupled Cluster energies can indeed be obtained at near DFT cost.
-
exploring the accuracy limits of local pair natural orbital Coupled Cluster theory
Journal of Chemical Theory and Computation, 2015Co-Authors: Dimitrios G Liakos, Manuel Sparta, Manoj K Kesharwani, Jan M L Martin, Frank NeeseAbstract:The domain based local pair natural orbital Coupled Cluster method with single-, double-, and perturbative triple excitations (DLPNO–CCSD(T)) is an efficient quantum chemical method that allows for Coupled Cluster calculations on molecules with hundreds of atoms. Because Coupled-Cluster theory is the method of choice if high-accuracy is needed, DLPNO–CCSD(T) is very promising for large-scale chemical application. However, the various approximations that have to be introduced in order to reach near linear scaling also introduce limited deviations from the canonical results. In the present work, we investigate how far the accuracy of the DLPNO–CCSD(T) method can be pushed for chemical applications. We also address the question at which additional computational cost improvements, relative to the previously established default scheme, come. To answer these questions, a series of benchmark sets covering a broad range of quantum chemical applications including reaction energies, hydrogen bonds, and other noncov...
Dimitrios G Liakos - One of the best experts on this subject based on the ideXlab platform.
-
is it possible to obtain Coupled Cluster quality energies at near density functional theory cost domain based local pair natural orbital Coupled Cluster vs modern density functional theory
Journal of Chemical Theory and Computation, 2015Co-Authors: Dimitrios G Liakos, Frank NeeseAbstract:The recently developed domain-based local pair natural orbital Coupled Cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) delivers results that are closely approaching those of the parent canonical Coupled Cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2–3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2–4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality. The computational effort for the Coupled Cluster step scales nearly linearly with system size. Since, in many instances, the Coupled Cluster step in DLPNO-CCSD...
-
is it possible to obtain Coupled Cluster quality energies at near density functional theory cost domain based local pair natural orbital Coupled Cluster vs modern density functional theory
Journal of Chemical Theory and Computation, 2015Co-Authors: Dimitrios G Liakos, Frank NeeseAbstract:The recently developed domain-based local pair natural orbital Coupled Cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) delivers results that are closely approaching those of the parent canonical Coupled Cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2-3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2-4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality. The computational effort for the Coupled Cluster step scales nearly linearly with system size. Since, in many instances, the Coupled Cluster step in DLPNO-CCSD(T) is cheaper or at least not much more expensive than the preceding Hartree-Fock calculation, it is useful to compare the method against modern density functional theory (DFT), which requires an effort comparable to that of Hartree-Fock theory (at least if Hartree-Fock exchange is part of the functional definition). Double hybrid density functionals (DHDF's) even require a MP2-like step. The purpose of this article is to evaluate the cost vs accuracy ratio of DLPNO-CCSD(T) against modern DFT (including the PBE, B3LYP, M06-2X, B2PLYP, and B2GP-PLYP functionals and, where applicable, their van der Waals corrected counterparts). To eliminate any possible bias in favor of DLPNO-CCSD(T), we have chosen established benchmark sets that were specifically proposed for evaluating DFT functionals. It is demonstrated that DLPNO-CCSD(T) with any of the three default thresholds is more accurate than any of the DFT functionals. Furthermore, using the aug-cc-pVTZ basis set and the LoosePNO default settings, DLPNO-CCSD(T) is only about 1.2 times slower than B3LYP. With NormalPNO thresholds, DLPNO-CCSD(T) is about a factor of 2 slower than B3LYP and shows a mean absolute deviation of less than 1 kcal/mol to the reference values for the four different data sets used. Our conclusion is that Coupled Cluster energies can indeed be obtained at near DFT cost.
-
exploring the accuracy limits of local pair natural orbital Coupled Cluster theory
Journal of Chemical Theory and Computation, 2015Co-Authors: Dimitrios G Liakos, Manuel Sparta, Manoj K Kesharwani, Jan M L Martin, Frank NeeseAbstract:The domain based local pair natural orbital Coupled Cluster method with single-, double-, and perturbative triple excitations (DLPNO–CCSD(T)) is an efficient quantum chemical method that allows for Coupled Cluster calculations on molecules with hundreds of atoms. Because Coupled-Cluster theory is the method of choice if high-accuracy is needed, DLPNO–CCSD(T) is very promising for large-scale chemical application. However, the various approximations that have to be introduced in order to reach near linear scaling also introduce limited deviations from the canonical results. In the present work, we investigate how far the accuracy of the DLPNO–CCSD(T) method can be pushed for chemical applications. We also address the question at which additional computational cost improvements, relative to the previously established default scheme, come. To answer these questions, a series of benchmark sets covering a broad range of quantum chemical applications including reaction energies, hydrogen bonds, and other noncov...
Ove Christiansen - One of the best experts on this subject based on the ideXlab platform.
-
communication a reduced space algorithm for the solution of the complex linear response equations used in Coupled Cluster damped response theory
Journal of Chemical Physics, 2013Co-Authors: Joanna Kauczor, Ove Christiansen, Patrick Norman, Sonia CorianiAbstract:We present a reduced-space algorithm for solving the complex (damped) linear response equations required to compute the complex linear response function for the hierarchy of methods: Coupled Cluster singles, Coupled Cluster singles and iterative approximate doubles, and Coupled Cluster singles and doubles. The solver is the keystone element for the development of damped Coupled Cluster response methods for linear and nonlinear effects in resonant frequency regions.
-
excited state Coupled Cluster methods
Wiley Interdisciplinary Reviews: Computational Molecular Science, 2012Co-Authors: Kristian Sneskov, Ove ChristiansenAbstract:We review Coupled Cluster (CC) theory for electronically excited states. We outline the basics of a CC response theory framework that allows the transfer of the attractive accuracy and convergence properties associated with CC methods over to the calculation of electronic excitation energies and properties. Key factors affecting the accuracy of CC excitation energy calculations are discussed as are some of the key CC models in this field. To aid both the practitioner as well as the developer of CC excited state methods, we also briefly discuss the key computational steps in a working CC response implementation. Approaches aimed at extending the application range of CC excited state methods either in terms of molecular size and phenomena or in terms of environment (solution and proteins) are also discussed. © 2011 John Wiley & Sons, Ltd.
-
Coupled-Cluster theory in a projected atomic orbital basis.
The Journal of chemical physics, 2006Co-Authors: Ove Christiansen, Pekka Manninen, Poul Jørgensen, Jeppe OlsenAbstract:We present a biorthogonal formulation of Coupled-Cluster (CC) theory using a redundant projected atomic orbital (PAO) basis. The biorthogonal formulation provides simple equations, where the projectors involved in the definition of the PAO basis are absorbed in the integrals. Explicit expressions for the Coupled-Cluster singles and doubles equations are derived in the PAO basis. The PAO CC equations can be written in a form identical to the standard molecular orbital CC equations, only with integrals that are related to the atomic orbital integrals through different transformation matrices. The dependence of Cluster amplitudes, integrals, and correlation energy contributions on the distance between the participating atomic centers and on the number of involved atomic centers is illustrated in numerical case studies. It is also discussed how the present reformulation of the CC equations opens new possibilities for reducing the number of involved parameters and thereby the computational cost.
-
full configuration interaction benchmarking of Coupled Cluster models for the lowest singlet energy surfaces of n2
Journal of Chemical Physics, 2000Co-Authors: Helena Larsen, Jeppe Olsen, Poul Jørgensen, Ove ChristiansenAbstract:The potential-energy curves for the X 1Σg+, a 1Πg, a′ 1Σu−, w 1Δu, c3 1Πu, and b 1Πu states of N2 have been investigated in full configuration interaction (FCI) and Coupled-Cluster response calculations. The equilibrium bond lengths, adiabatic excitation energies, and harmonic frequencies have been obtained with the Coupled-Cluster singles model (CCS), an approximate Coupled-Cluster singles and doubles model (CC2), the Coupled-Cluster singles and doubles model (CCSD), and an approximate Coupled-Cluster singles, doubles, and triples model (CC3), and subsequently compared to FCI results. The weak and strong features of the Coupled-Cluster models are discussed and illustrated. Overall, improvements towards FCI are obtained in the hierarchy CCS–CC2–CCSD–CC3. CC3 is always consistently better than CCSD, and for all the considered spectroscopic constants CC3 provides excellent results. Examples where the CC3 model fails are also given. The noniterative triples model, CCSDR(3), is compared to the iterative tripl...
-
the second order approximate Coupled Cluster singles and doubles model cc2
Chemical Physics Letters, 1995Co-Authors: Ove Christiansen, Henrik Koch, Poul JørgensenAbstract:Abstract An approximate Coupled Cluster singles and doubles model is presented, denoted CC2. The CC2 total energy is of second-order Moller-Plesset perturbation theory (MP2) quality. The CC2 linear response function is derived. Unlike MP2, excitation energies and transition moments can be obtained in CC2. A hierarchy of Coupled Cluster models, CCS, CC2, CCSD, CC3, CCSDT etc., is presented where CC2 and CC3 are approximate Coupled Cluster models defined by similar approximations. Higher levels give increased accuracy at increased computational effort. The scaling of CCS, CC2, CCSD, CC3 and CCSDT is N4, N5, N6, N7 and N8, respectively where N is th the number of orbitals. Calculations on Be, N2 and C2H4 are performed and results compared with those obtained in the second-order polarization propagator approach SOPPA.
Gustavo E. Scuseria - One of the best experts on this subject based on the ideXlab platform.
-
projected Coupled Cluster theory optimization of Cluster amplitudes in the presence of symmetry projection
Journal of Chemical Physics, 2018Co-Authors: Yiheng Qiu, Thomas M Henderson, Jinmo Zhao, Gustavo E. ScuseriaAbstract:Methods which aim at universal applicability must be able to describe both weak and strong electronic correlation with equal facility. Such methods are in short supply. The combination of symmetry projection for strong correlation and Coupled Cluster theory for weak correlation offers tantalizing promise to account for both on an equal footing. In order to do so, however, the Coupled Cluster portion of the wave function must be optimized in the presence of the symmetry projection. This paper discusses how this may be accomplished, and shows the importance of doing so for both the Hubbard model Hamiltonian and the molecular Hamiltonian, all with a computational scaling comparable to that of traditional Coupled Cluster theory.
-
attenuated Coupled Cluster a heuristic polynomial similarity transformation incorporating spin symmetry projection into traditional Coupled Cluster theory
Molecular Physics, 2017Co-Authors: John A Gomez, Thomas M Henderson, Gustavo E. ScuseriaAbstract:In electronic structure theory, restricted single-reference Coupled Cluster (CC) captures weak correlation but fails catastrophically under strong correlation. Spin-projected unrestricted Hartree-F...
-
merging symmetry projection methods with Coupled Cluster theory lessons from the lipkin model hamiltonian
Journal of Chemical Physics, 2017Co-Authors: Jacob M Wahlenstrothman, Yiheng Qiu, Thomas M Henderson, Jinmo Zhao, Jorge Dukelsky, Matthew R Hermes, Matthias Degroote, Gustavo E. ScuseriaAbstract:Coupled Cluster and symmetry projected Hartree-Fock are two central paradigms in electronic structure theory. However, they are very different. Single reference Coupled Cluster is highly successful for treating weakly correlated systems but fails under strong correlation unless one sacrifices good quantum numbers and works with broken-symmetry wave functions, which is unphysical for finite systems. Symmetry projection is effective for the treatment of strong correlation at the mean-field level through multireference non-orthogonal configuration interaction wavefunctions, but unlike Coupled Cluster, it is neither size extensive nor ideal for treating dynamic correlation. We here examine different scenarios for merging these two dissimilar theories. We carry out this exercise over the integrable Lipkin model Hamiltonian, which despite its simplicity, encompasses non-trivial physics for degenerate systems and can be solved via diagonalization for a very large number of particles. We show how symmetry projection and Coupled Cluster doubles individually fail in different correlation limits, whereas models that merge these two theories are highly successful over the entire phase diagram. Despite the simplicity of the Lipkin Hamiltonian, the lessons learned in this work will be useful for building an ab initio symmetry projected Coupled Cluster theory that we expect to be accurate in the weakly and strongly correlated limits, as well as the recoupling regime.
-
merging symmetry projection methods with Coupled Cluster theory lessons from the lipkin model hamiltonian
Journal of Chemical Physics, 2017Co-Authors: Jacob M Wahlenstrothman, Yiheng Qiu, Thomas M Henderson, Jinmo Zhao, Jorge Dukelsky, Matthew R Hermes, Matthias Degroote, Gustavo E. ScuseriaAbstract:Coupled Cluster and symmetry projected Hartree-Fock are two central paradigms in electronic structure theory. However, they are very different. Single reference Coupled Cluster is highly successful for treating weakly correlated systems but fails under strong correlation unless one sacrifices good quantum numbers and works with broken-symmetry wave functions, which is unphysical for finite systems. Symmetry projection is effective for the treatment of strong correlation at the mean-field level through multireference non-orthogonal configuration interaction wavefunctions, but unlike Coupled Cluster, it is neither size extensive nor ideal for treating dynamic correlation. We here examine different scenarios for merging these two dissimilar theories. We carry out this exercise over the integrable Lipkin model Hamiltonian, which despite its simplicity, encompasses non-trivial physics for degenerate systems and can be solved via diagonalization for a very large number of particles. We show how symmetry project...
-
attenuated Coupled Cluster a heuristic polynomial similarity transformation incorporating spin symmetry projection into traditional Coupled Cluster theory
arXiv: Chemical Physics, 2017Co-Authors: John A Gomez, Thomas M Henderson, Gustavo E. ScuseriaAbstract:In electronic structure theory, restricted single-reference Coupled Cluster (CC) captures weak correlation but fails catastrophically under strong correlation. Spin-projected unrestricted Hartree-Fock (SUHF), on the other hand, misses weak correlation but captures a large portion of strong correlation. The theoretical description of many important processes, e.g. molecular dissociation, requires a method capable of accurately capturing both weak- and strong correlation simultaneously, and would likely benefit from a combined CC-SUHF approach. Based on what we have recently learned about SUHF written as particle-hole excitations out of a symmetry-adapted reference determinant, we here propose a heuristic Coupled Cluster doubles model to attenuate the dominant spin collective channel of the quadratic terms in the Coupled Cluster equations. Proof of principle results presented here are encouraging and point to several paths forward for improving the method further.
