Abstract
We implemented the state-specific Brillouin–Wigner coupled-cluster method for the complete model space spanned by four reference configurations generated by two electrons in two active orbitals. We applied the method (together with the previously suggested a posteriori size-extensivity correction) to the calculation of spectroscopic constants of the IBr molecule, using averaged relativistic effective core potential.
Highlights
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
The method so developed was general in respect to the number of reference configurations, its implementation [4] in the ACES II program [10] was limited to two-reference cases, where HOMO and LUMO orbitals have different spatial symmetry and only two closed-shell configurations can contribute to the wave function
In order to be able to treat molecules with quasidegenerate HOMO and LUMO orbitals with the same spatial symmetry, we extended our original implementation of the BWCCSD method for open shell reference configurations
Summary
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. In order to be able to treat molecules with quasidegenerate HOMO and LUMO orbitals with the same spatial symmetry, we extended our original implementation of the BWCCSD method for open shell reference configurations. The correction in its original implementation [6] was confined to two-reference cases In this paper it has been generalized and calculated for four reference configurations. The early measurements of spectra of IBr in visible [12] and ultraviolet [13] have found bands corresponding to transitions to higher electronic states Further studies confirmed these findings [14] and gave refined values of rotational and vibrational constants from high resolution far infrared absorption spectra [15]. Subsequent ab initio studies with inclusion of relativistic effects provided values of spectroscopic [11,22,23] and electric [22,24,25] properties
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