In pursuit of the ab initio limit for conformational energy prototypes

The convergence of ab initio predictions to the one- and $n$ -particle limits has been systematically explored for several conformational energy prototypes: the inversion barriers of ammonia, water, and isocyanic acid, the torsional barrier of ethane, the $\mathrm{E/Z}$ rotamer separation of formic acid, and the barrier to linearity of silicon dicarbide. Explicit ab initio results were obtained with atomic-orbital basis sets as large as $\text{[7s6p5d4f3g2h1i/6s5p4d3f2g1h]}$ and electron correlation treatments as extensive as fifth-order Mo/ller–Plesset perturbation theory (MP5), the full coupled-cluster method through triple excitations (CCSDT), and Brueckner doubles theory including perturbational corrections for both triple and quadruple excitations [BD(TQ)]. Subsequently, basis set and electron correlation extrapolation schemes were invoked to gauge any further variations in arriving at the ab initio limit. Physical effects which are tacitly neglected in most theoretical work have also been quantified by computations of non-Born–Oppenheimer (BODC), relativistic, and core correlation shifts of relative energies. Instructive conclusions are drawn for the pursuit of spectroscopic accuracy in theoretical conformational analyses, and precise predictions for the key energetic quantities of the molecular prototypes are advanced.