The description of N2 and F2 potential energy surfaces using multireference coupled cluster theory

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Abstract
The ground state potential energy surfaces (PES’s) for diatomic nitrogen and fluorine are examined using a version of our recently published linearized multireference coupled cluster method (MR‐LCCM). Comparison calculations employing a variety of standard ab initio techniques such as single reference configuration interaction singles and doubles (CISD), many‐body perturbation theory (MBPT), coupled cluster single and doubles (CCSD), and multireference (MR)‐CISD were also performed. In addition, the PES’s were also investigated using the newly developed CCSDT‐1 method, which includes the dominant effect of T̂3. These single reference procedures fail in various ways (with the possible exception of the CCSDT‐1 method), while the MR‐LCCM method is shown to compare favorably to the more traditional MR‐CI techniques. Like the MR‐CIs, the MR‐LCCM energy curves dissociate correctly and the two are nearly parallel.