Ivan Hubač

Assessment of the single-root multireference Brillouin–Wigner coupled- cluster method: Test calculations on CH2, SiH2, and twisted ethylene

Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which belongs to a broad family of state-selective multireference coupled-cluster methods, has been implemented in the ACES II program package at the CCSD level of approximation. The method represents a new approach to quasidegenerate problems, which is able to continuously switch between the single-reference CC in a nondegenerate situation and the Hilbert-space MRCC in a degenerate case. An assessment of the method has been carried out by means of a comparison with the full configuration interaction (CI) treatments of CH2, SiH2, and twisted ethylene diradicals. The problem of size-extensivity is discussed.

Multireference Brillouin-Wigner Coupled-Cluster Theory. Single-root approach.

Recently developed Brillouin-Wigner coupled-cluster theory [I. Hubac and P. Neogrady, Phys. Rev. A 50 , 4558 (1994)] is extended to a multireference case using the Hilbert space approach. We formulate the so-called single-root (one-state or state-specific) version which deals with one state at a time while employing a multiconfigurational reference wave function. Employing the Hilbert space approach to the wave operator, we present an explicit form for cluster amplitudes in a spin-orbital form within the CCSD approximation; i.e. coupled-cluster method truncated at the single and double excitation level. The method is applied to a trapezoidal H4 model system with the use of a two-determinant reference space and the results are compared with the full configuration interaction as well as other correlated multireference techniques. The method provides a balanced description of the ground state in both quasidegenerate and nondegenerate regions and deviations from the full configuration interaction energies do not exceed 0.6 mHartree.

Single-root multireference Brillouin-Wigner coupled-cluster theory: Applicability to the F2 molecule

Recently developed single-root multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory, which deals with one state at a time while employing a multiconfigurational reference wave function, is applied to the ground state of the F2 molecule using a two-determinant reference space at the level of the CCSD approximation. The method represents a brand-new coupled-cluster (CC) approach to quasidegenerate problems which combines merits of two theories: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasidegenerate case. The method is able to switch itself from a nondegenerate to a fully degenerate case in a continuous manner, providing thus smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC approaches, it does not contain the so-called coupling terms and completely reduces to the standard single-reference CC method in a highly nondegenerate region. Using a [4s,3p,1d] and [4s,3p,2d,1f ] basis sets, the calculated potential ener...

Multireference coupled-cluster calculations on the energy of activation in the automerization of cyclobutadiene: Assessment of the state-specific multireference Brillouin–Wigner theory

Hilbert-space state-universal multireference coupled-cluster (MR CC) data on cyclobutadiene [A. Balkova and R. J. Bartlett, J. Chem. Phys. 101, 8972 (1994)] were used as a benchmark for testing our recently developed state-specific (single-root) multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory. For the energy of activation in the automerization of cyclobutadiene (i.e., the energy difference between the square and rectangular structures) at the CCSD/[3s2p1d/2s] level of theory, our MR BWCCSD method gives the value of 6.2 kcal/mol, compared to 6.5 kcal/mol given by MR CCSD. With the cc-pVDZ and cc-pVTZ/cc-pVDZ basis sets, the MR BWCCSD activation barrier is 6.4 and 7.0 kcal/mol, respectively. The effect of the triple excitations [in MR CCSD(T)] and of the frozen core approximation were estimated previously to be below 0.1 kcal/mol and in the opposite direction. This shows the way of how to arrive at a more accurate automerization barrier in future calculations: extension of the basis set see...

On the use of Brillouin-Wigner perturbation theory for many-body systems

The use of Brillouin-Wigner perturbation theory in describing many-body systems is critically re-examined.

State-specific Brillouin–Wigner multireference coupled cluster study of the F2 molecule: assessment of the a posteriori size-extensivity correction ☆

Abstract We tested a posteriori correction suggested previously for the state-specific multireference Brillouin–Wigner coupled-cluster singles and doubles (MR BW CCSD) theory, to eliminate its size-extensivity error. The correction was applied to a two-reference BW CCSD model by using the cc-pVXZ basis sets ( X =2,3,4) and it was tested by calculating the spectroscopic constants of the F 2 molecule.

Brillouin-Wigner Methods for Many-Body Systems

The Brillouin–Wigner many-body problem in atomic and molecular physics and in quantum chemistry is described. The use of coupled cluster expansions, configuration interaction and perturbation series is considered both for the single-reference function case and for those cases requiring the use of a multi-reference formalism.

Calculation of singlet and triplet excitation energies, ionization potentials, and electron affinities of closed-shell systems by many-body Rayleigh-Schrödinger perturbation theory

A method for the calculation of singlet and triplet excitation energies, ionization potentials, and electron affinities by the many-body Rayleigh-Schrodinger perturbation theory is elaborated. In this method the excitation energies, ionization potentials, and electron affinities are given by an infinite sum of terms, each of which may be interpreted using the diagrammatic technique. Application of this method to systems described by a π-electron PPP-Hamiltonian is carried out.

On the ground states of CaC and ZnC: A multireference Brillouin–Wigner coupled cluster study

We test the recently developed state-specific multireference Brillouin–Wigner coupled cluster (MRBWCCSD) method against the single reference CCSD method by examining theoretically the competing X 3Σ− and 5Σ− states of the (experimentally unknown) isovalent calcium and zinc carbide diatomics (CaC, ZnC). At the CCSD level, CaC is “incorrectly” predicted to have a ground 5Σ− state; however, the MRBWCCSD treatment restores the correct state ordering, and improves significantly the energetics for both molecules. Further comparison with various single- and multireference treatments shows that the latter are absolutely necessary for obtaining meaningful results for the ground states in both molecules.

Spin adapted restricted open shell coupled cluster theory. Linear version

We describe the linear version of the spin adapted coupled cluster theory with all single and double excitations for open shell systems in restricted Hartree–Fock formalism. The method is based on the partitioning of the Hamiltonian developed by Hubac and Carsky [I. Hubac and P. Carsky, Phys. Rev. A 22, 2392 (1980)] in restricted Roothaan open shell formalism. Correlation energy of some simple molecular systems is calculated.