Unitary group based open-shell coupled cluster theory: Application to van der Waals interactions of high-spin systems

Abstract

Articles you may be interested inDensity functional theory study of the interaction of vinyl radical, ethyne, and ethene with benzene, aimed to define an affordable computational level to investigate stability trends in large van der Waals complexes J. Chem. Phys. 139, 244306 (2013); 10.1063/1.4846295 Symmetry-adapted perturbation theory based on unrestricted Kohn-Sham orbitals for high-spin open-shell van der Waals complexes J. Chem. Phys. 137, 164104 (2012); 10.1063/1.4758455 Static polarizability surfaces of the van der Waals complex CH 4 -N 2The performance of the unitary group approach ͑UGA͒ based coupled cluster singles and doubles ͑CCSD͒ method in application to van der Waals interactions involving high-spin open-shell systems is examined. The tested approach is fully spin-adapted in the sense that any intermediate quantity appearing in the formulation of the theory is free from spin contamination contributions. Interaction energies are computed within the supermolecular approach and corrected for the basis set superposition error. Several methods of solving UGA CCSD equations are used with the emphasis on iterative processes based on the Hamiltonian partitionings employed in the spin-restricted many-body perturbation theories. Test calculations are performed for the ground states of HeLi, H 2 Li, and for the excited a 3 ⌺ u ϩ state of Li 2 . The UGA CCSD interaction energies are compared with those computed using the spin-unrestricted and valence universal coupled cluster methods, spin-restricted and spin-unrestricted many-body perturbation expansions, and the full configuration interaction method. The obtained results show that the UGA CCSD method performs very well in applications to weakly bound open-shell systems, giving results as good or better than other open-shell coupled cluster approaches.

Higher excitations in coupled-cluster theory

“…9 Some results have been reported about the automatized generation of codes of CC methods utilizing second-quantization. [10][11][12] Some mathematical approaches have also been worked out to obtain the full set of solutions of the CC problem [13][14][15] and were applied for four hydrogen atoms at various geometries. [15][16][17][18][19] Explicit inclusion of higher ͑5, 6,...͒ excitations into the CC equations, however, has not yet been done due to the tremendous difficulties in putting down the corresponding equations.…”

Multireference Coupled-Cluster Methods: Recent Developments

“…We payed only a cursory, if any, attention to those methods which are surveyed in greater detail in other chapters of this monograph, and passed over those SR approaches that exploit, either explicitly or implicitly, a multi-determinantal reference, such as various spin-adapted approaches (e.g., the CAUGA based CC [36,37,74], spin-flip approaches [210][211][212][213], etc.). We shall now try to assess the merits and limitations of the above reviewed methods by relying on the available results.…”

“…There exists nowadays a whole plethora of such methods to which we turn our attention in this chapter. We should also mention, at least in passing, a version exploiting, in general, a multi-determinantal reference of the Clifford algebra unitary group approach (CAUGA) [69][70][71][72][73] that can handle open-shell singlet, doublet, and triplet states and is fully spin adapted [36,37,74].…”

“…Implementations of higher excitations with proper computational scaling have been developed in two ways, both involving computer manipulation of expressions, pioneered in the MBPT [33,34] and CC [35][36][37] formalisms already some time ago. The task of diagram generation and factorization of the resulting expressions is passed to the computer in both approaches, however, they differ in the way how the resulting tensor contractions are evaluated.…”

Trends in low-lying electronic states of XH2 (X=Li, Na, K)

“…For instance, the ubiquity of atomic and molecular hydrogen throughout the interstellar media of the early (and present) universe would suggest that primordial chemistry, and hence the early chemical evolution of the universe, may have been dominated by the formation/ decomposition processes of simple metal hydrides [1][2][3]. There has therefore been extensive theoretical investigation into the nature of LiH 2 [4][5][6][7][8][9][10][11][12]. In addition, due to the relatively high hydrogen mass percentage, small lithium hydride species are ideal hydrogen storage candidates [6].…”