Jozef Mášik

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 Brillouin—Wigner Coupled-Cluster Theory: Hilbert Space Approach

Recently developed Brillouin—Wigner coupled-cluster theory (I. Hubac and P. Neogrady: Phys. Rev. A 50, 4558 (1994)) is extended to the multireference case using the Hilbert space approach. We present two computational strategies, namely the ‘multi-root’ as well as ‘single-root’ formulations of the multireference Brillouin—Wigner coupled-cluster (MR BWCC) theory and we discuss their relations to existing multireference coupled-cluster (MR CC) theories. We show that the multi-root formulation of the MR BWCC theory is equivalent to existing MR CC theories while the single-root formulation does not have such an analogy. Using the Hilbert space approach to the wave operator we present basic equations of the MR BWCCSD approximation, i.e. a coupled-cluster method with the inclusion of singly and doubly excited cluster components.

Single-root multireference Brillouin-Wigner coupled-cluster theory. Rotational barrier of the N2H2 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 study the rotational barrier of the N2H2 molecule. The method represents a brand new coupled-cluster (CC) approach to quasi-degenerate problems which combines merits of two approaches: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasi-degenerate case. The method is able to switch itself from a nondegenerate to a fully degenerate case in a continuous manner, thus providing smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC theory, it does not contain the so-called coupling terms and in a highly nondegenerate case it reduces to a standard single-reference CC method. In order to better judge the abilitiesof our new approach, we study the rotation barrier of the N2H2 molecule at the CCSD level and the results are compared...