Single-root multireference Brillouin—Wigner coupled-cluster theory. Rotational barrier of the N2H2 molecule

Accurate MRCI study of ground-state N2H2 potential energy surface

“…Some of these studies focused on the determination of accurate geometries, frequencies and relative stabilities of three local minima: trans-, cis-, and iso-N 2 H 2 [3,7,[14][15][16][17][18][19]11,10]. Moreover, there has been considerable theoretical work [2,1,9,4,8,6,5,12,13] on the determination of the barriers involved in the diimide isomerization or formation processes, often for testing the accuracy of theoretical methods that are used to study chemical reactions [13,12,4,6]. All published work agrees on the structures of the above minima (trans-, cis-, and iso-) for N 2 H 2 , although the agreement for transition states (TSs) stands only for the TS associated to the trans-iso conversion.…”

“…There have been several theoretical [1][2][3][4][5][6][7][8][9][10][11][12][13] and experimental [14][15][16][17][18][19] studies of the title system (a more complete list of earlier theoretical and experimental work is given in Refs. [3,2]).…”

Benchmark ab initio thermochemistry of the isomers of diimide, N2H2, using accurate computed structures and anharmonic force fields

“…[27,28]) calculations on the relative stability of the trans-diazene, cis-diazene, and isodiazene isomers have consistently shown a stability ordering trans > cis > iso, as well as high (in excess of 40 kcal/mol) isomerization barriers between the isomers. (Very recently, the rigid cis-trans rotation of the molecule was proposed and studied [29] as a test case for a new multireference coupled cluster method [29], the transition state being a 'real-life' alternative for H 4 as an essentially perfect two-configuration reference problem. )…”

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

“…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.…”