Coupled cluster

About: Coupled cluster is a research topic. Over the lifetime, 6280 publications have been published within this topic receiving 301055 citations.

Low-order scaling local electron correlation methods. V. Connected triples beyond (T): Linear scaling local CCSDT-1b

Abstract: A new O(N ) method for the iterative treatment of connected triple substitutions in the framework of local coupled cluster theory is introduced here, which is the local equivalent of the canonical CCSDT-1b method. The effect of the triple substitutions is treated in a self-consistent manner in each coupled cluster iteration. As for the local (T) method presented earlier in this series the computational cost of the method scales asymptotically linear with molecular size. The additional computational burden due to the involvement of triples in each coupled cluster iteration hence is not nearly as dramatic as for the parental canonical method, where it implies an increase in the computational complexity of the coupled cluster iteration from O(N6) to O(N7). The method has certain advantages in comparison to the perturbative a posteriori treatment of connected triples (T) for cases where static correlation effects start to play a role. It is demonstrated that molecules with about 100 atoms and 1000 basis funct...

Towards a force field based on density fitting

Abstract: Total intermolecular interaction energies are determined with a first version of the Gaussian electrostatic model (GEM-0), a force field based on a density fitting approach using s-type Gaussian functions. The total interaction energy is computed in the spirit of the sum of interacting fragment ab initio (SIBFA) force field by separately evaluating each one of its components: electrostatic (Coulomb), exchange repulsion, polarization, and charge transfer intermolecular interaction energies, in order to reproduce reference constrained space orbital variation (CSOV) energy decomposition calculations at the B3LYP/aug-cc-pVTZ level. The use of an auxiliary basis set restricted to spherical Gaussian functions facilitates the rotation of the fitted densities of rigid fragments and enables a fast and accurate density fitting evaluation of Coulomb and exchange-repulsion energy, the latter using the overlap model introduced by Wheatley and Price [Mol. Phys. 69, 50718 (1990)]. The SIBFA energy scheme for polarization and charge transfer has been implemented using the electric fields and electrostatic potentials generated by the fitted densities. GEM-0 has been tested on ten stationary points of the water dimer potential energy surface and on three water clusters (n=16,20,64). The results show very good agreement with density functional theory calculations, reproducing the individual CSOV energy contributions for a given interaction as well as the B3LYP total interaction energies with errors below kBT at room temperature. Preliminary results for Coulomb and exchange-repulsion energies of metal cation complexes and coupled cluster singles doubles electron densities are discussed.

Performance of CCSDT for diatomic dissociation energies

Abstract: Calculations of 11 diatomic dissociation energies with coupled cluster theory through iterative triple excitations highlight both the strength and limitations of this method. By combining very large basis sets (through septuple zeta in some cases) and complete basis set extrapolations with corrections for core/valence correlation, scalar relativistic and atomic/molecular spin–orbit effects, it was possible to achieve excellent agreement with experiment in most cases. However, for C2 and CN the extent of the multiconfigurational nature of the molecules caused problems for the single configuration-based couple cluster methods. In the worse case, the inclusion of iterative triples resulted in a change with respect to the perturbative triples result which was of the opposite sign to the full configuration interaction change. This work emphasizes the difficulties in achieving uniform chemical accuracy even for ground state, first and second row diatomics.

A multireference coupled‐cluster study of the ground state and lowest excited states of cyclobutadiene

Abstract: The electronic structure of the ground state and several low‐lying excited states of cyclobutadiene are studied using the new state‐universal multireference coupled‐cluster method with single and double excitations (MR‐CCSD) augmented by a noniterative inclusion of the triple excitations [MR‐CCSD(T)]. Two possible ground state configurations are examined, namely the square and the distorted rectangular geometries, and the multireference coupled‐cluster energy barrier for the interconversion between the two rectangular ground state structures is estimated to be 6.6 kcal mol−1 compared with the best theoretical value, 6.4 kcal mol−1 obtained using the highly accurate coupled‐cluster method with full inclusion of the triple excitations (CCSDT). The ordering of electronic states for the square geometry is determined, with the ground state singlet being located 6.9 kcal mol−1 below the lowest triplet electronic state. We also examine the potential energy surface for the interconversion between the two equivalent second‐order Jahn–Teller rhombic structures for the first excited singlet state. When comparing the MRCC energies with the results provided by various single‐ and multireference correlation methods, the critical importance of including both the dynamic and nondynamic correlation for a qualitatively correct description of the electronic structure of cyclobutadiene is emphasized. We also address the invariance properties of the present MRCC methods with respect to the alternative selections of reference orbital spaces.

State-Specific Multi-Reference Coupled Cluster Formulations: Two Paradigms

Abstract: The traditional multi-reference coupled cluster (MRCC) methods are based on effective hamiltonian formalism and often suffer from the problem of intruders. A state-specific MRCC approach, focusing on only one state, offers the attractive possibility of avoiding intruders while at the same time incorporating the nondynamical correlation in a size-extensive manner. In this paper we discuss two alternative paradigms which allow us to achieve this goal. The first, to be called the decontracted description, we deliberately retain certain linearly dependent cluster amplitudes and allow the combining coefficients of the reference determinants to be updated to their values for the exact function. The presence of the linearly dependent cluster amplitudes requires imposition of suitable sufficiency conditions, which are invoked in a manner which naturally ensures size-extensivity. In the second approach, to be called the contracted description, we would generate a cluster expansion with respect to the entire reference function consisting of a combination of reference determinants and retain only the linearly independent cluster-amplitudes in the cluster expansion. For an efficient implementation of the formalism, we shall introduce the notion of extended normal ordering and an analogue of Wick's theorem which uses the entire reference function as the multi-determinantal analogue of the vacuum. This necessarily imposes the restriction that the combining coefficients appearing in the reference function have to remain frozen in the equations for the cluster amplitudes. Relaxation of the coefficients can be acheived only after the cluster-amplitudes with the current coefficients are solved.