An ab initio study of the FCP potential energy surface

Abstract The potential energy surface of FCP was studied with high-level single-reference electron correlated optimization. It was found that linear FPC is a potential energy minimum at the RMP2 and RMP4 levels of theory but a maximum at UMP2, RPM3, UMP3 and UMP4. Moreover, various numbers of minima (Min) and saddle points (TSs) were found on the restricted and unrestricted MP n ( n = 2–4) surfaces: 3 Min and 2 TSs on RMP2, 3 Min and 3 TSs on UMP2, 2 Min and 2 TSs on RMP3, 3 Min and 3 TSs on UMP3, 3 Min and 2 TSs on RMP4, and 3 Min and 3 TSs on UMP4. However, at the more reliable quadratic CI and Brueckner doubles levels of theory, only two minima, corresponding to linear FCP and bent FPC (bond angle about 110 °), and two saddle points were found on the potential energy surface.