Application of the uncoupled state-specific multireference coupled cluster method to a weakly bonded system: exploring the ground state Be2

摘要: Recently, Mukherjee and co-workers (2008 Chem. Phys. 349, 115) have proposed the uncoupled state-specific multireference coupled cluster approach (UC-SSMRCC), attuned particularly for systems to provide a relaxed description of nondynamical correlation (stemming from quasidegeneracy) in presence dynamical correlation. By invoking an analogue anonymous parentage approximation coupling terms (SS-MRCC) theory one obtains UC-SSMRCC method without significantly sacrificing accuracy simultaneously retains all key features parent theory. For this very reason calculations using large basis sets are computationally more feasible even with inclusion connected triples than one. As far as our knowledge is concerned, no application either SS-MRCC or has been reported weakly bound date. Hence, we performed study dimer (say X1Σ+ Be2) by singles-, doubles-, triples-1a (SDT-1a) level approximation. Because strong near-degeneracy 2s 2p atomic orbitals, Be2 suitable benchmark problem quantum chemistry critical test new theoretical methods procedures. This reveals that quality critically depends on truncation operators (and also choice set) delicately poised treatment dynamic nondynamic effects. Our computed spectroscopic constants acceptably good agreement other previously current generation recovery estimates, indicating SDT-1a capable achieving qualitatively correct energy surface smallest reference space. The scheme providing estimates almost deterioration vis-a-vis SS-MRCC. Accordingly, it hoped realm will be increased.