State-Specific Brillouin−Wigner Multireference Coupled Cluster Study of the Singlet−Triplet Separation in the Tetramethyleneethane Diradical

Abstract

The potential energy curves for the twisting of tetramethyleneethane in its lowest singlet and triplet states were calculated by the state-specific two-reference Brillouin-Wigner coupled-cluster method with single and double excitations. The calculated potential energy curves are essentially the same as those obtained by the two-determinant CCSD method, and they are also in agreement with the previously reported density functional theory results. Our data bring support for the previously suggested interpretation of experimental data on tetramethyleneethane in the gas phase and in the matrix.

Mean-field resonance-theoretic view of benzenoid networks

“…The 2/3 of an unpaired electron on each terminal site cannot be paired to one another under the alternancy-pairing condition (0) of pairing solely between sites of the same type (starred or unstarred), so that there are 2 fully unpaired spins and the ground state is a spin triplet. The result here also agrees largely with numerous ab initio quantum-chemical computations 39,40,41,42,43,44,45 , as well as experiment 46,47,48 .…”

The State-Universal Multi-Reference Coupled-Cluster Theory: An Overview of Some Recent Advances

“…Alternative ways of simplifying the SUMRCCSD scheme are possible by considering the state-selective methods employing the Jeziorski-Monkhorst ansatz (cf., e.g., Refs. [30][31][32][33][34][35][36][37]. Although these new methods are highly promising, in this paper we focus on simplifying the original SUMRCC method within its conventional effective Hamiltonian formulation, which is exactly what the SUMRCCSD(1) approximation offers by reducing the number of cluster amplitudes that need to be determined by the iterative procedure and by reducing the number of nonlinear equations that need to be solved.…”

“…The direct use of the final energy expressions in CC calculations, as is done in our MMCC theory [7,[118][119][120][121][122][123] and its multi-reference extension discussed here and in Ref. 117, which may result in the introduction of unlinked terms, is also exploited in the Brillouin-Wigner MRCC method [32][33][34][35][36][37]. As in the approximate MMCC case, the Brillouin-Wigner MRCC approach is not size extensive.…”

“…In this paper, we focus on the genuine multi-reference CC (MRCC) theories, which are based on the concept of effective Hamiltonian acting in a multi-dimensional reference or model space [4,6,[8][9][10][11][12] and which use the exponential CC ansatz to parametrize the solutions of the generalized Bloch equation [8]. The state-specific (SS) MRCC methods [4,6], including the active-space CC approaches of Adamowicz, Piecuch, Bartlett, and their collaborators [13][14][15][16][17][18][19][20][21][22][23][24][25][26] and their more recent excited-state extensions [27][28][29], the SSMRCC approach of Mahapatra et al [30,31], and the highly promising Brillouin-Wigner MRCC approach [32][33][34][35][36][37], will not be discussed in this work, although the similarity of the wave function ansatz used in the genuine state-universal MRCC theory [4,6,, considered in this paper, and the form of the wave function exploited by the SSMRCC and Brillouin-Wigner MRCC approaches of Refs. 30, 31 and 32-37, respectively, should be noted.…”

Four-Reference State-Specific Brillouin-Wigner Coupled-Cluster Method: Study of the IBr Molecule

“…This paper is a continuation of our previous studies [1][2][3][4][5][6][7][8][9] on the development of a multi-reference coupled-cluster (MRCC) method that would be free of the problem of intruder states and that would be amenable to treatment of systems requiring more than two reference configurations. Avoidance of intruder states was achieved by using the Brillouin-Wigner (BW) resolvent, and simplicity of the method and feasibility of calculations were achieved by subjecting the BWCCSD method to a state-specific form [2][3][4].…”