Quasi-variational coupled cluster theory

Abstract

Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications JCP: BioChem. Phys. 6, 01B616 (2012) Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications J. Chem. Phys. 136, 044320 (2012) A theoretical study of the CX2N radicals (X = F, Cl, Br): The effect of halogen substitution on structure, isomerization, and energetics J. Chem. Phys. 136, 044316 (2012) Molecular properties via a subsystem density functional theory formulation: A common framework for electronic embedding J. Chem. Phys. 136, 044104 (2012) Statistical thermodynamics of 1-butanol, 2-methyl-1-propanol, and butanal J. Chem. Phys. 136, 034306 (2012) Additional information on J. Chem. Phys. We extend our previous work on the construction of new approximations of the variational coupled cluster method. By combining several linked pair functional transformations in such a way as to give appropriately balanced infinite-order contributions, in order to approximate eT †Ĥ eT L well at all orders, we formulate a new quantum chemical method, which we name quasi-variational coupled cluster. We demonstrate this method to be particularly robust in the regime of strong static electron correlation, improving significantly on our earlier approximate variational coupled cluster approach.

Accurate prediction of diradical chemistry from a single-reference density-matrix method: Model application to the bicyclobutane to gauche-1,3-butadiene isomerization

“…Recently, an O(r 4 )-scaling p2-RDM method 53,54 has been developed through tensor expansions of the excitations 2 T and the twoelectron integrals in the reduced Hamiltonian 2 K. Several pair correlation methods related to the p2-RDM method have also recently been developed. [55][56][57] …”

Analytic energy gradients for orbital‐optimized MP3 and MP2.5 with the density‐fitting approximation: An efficient implementation

“…Orbital optimized post‐Hartree–Fock (HF) methods, such as the coupled‐cluster doubles (CCD), active space and excited state CC methods, the second‐order perturbation theory (MP2), the third‐order perturbation theory (MP3), the coupled pair functional theories, the linearized coupled‐cluster doubles (LCCD), and the density‐cumulant functional theory (DCFT), have been reported in previous studies. Triples corrections for the orbital‐optimized CCD method have also been implemented . In the previous studies it has been shown that the OCC and OMP methods are quite beneficial for chemical systems with problematic electronic structures, such as symmetry‐breaking problems, transition states, free radicals, bond‐breaking problems, weak interactions, and predictions of the chemical reactivity .…”

Statistical analysis of activation and reaction energies with quasi-variational coupled-cluster theory

“…The approach retains the N 6 cost scaling of CCSD, but with an increased prefactor, because of additional matrix transformations, and additional N 5 work associated with multiple integral transformations arising from the orbital variations. For full details, see [11]. In previous publications, the QV methods compared favourably with multireference methods where CCSD(T) has failed dramatically [13].…”

“…Over the past few years, several single-reference methods has been developed to tackle inherently multireference systems [9,10]. One such family of methods is quasi-variational coupledcluster doubles, hereafter collectively denoted as the QV methods [11,12]. At the single and double excitation levels, standard CCSD is replaced by the 'quasi-variational' approximation to variational coupled cluster with double excitations, combined with variational optimisation of the energy functional with respect to variations in the reference orbitals (OQVCCD).…”