Analytical fittings for the global potential energy surface of the ground state of methylene

The global potential energy surface (PES) corresponding to the dissociation reaction of the ground state of methylene (CH₂) is studied with the coupled-cluster method with single, double and perturbative triplet excitations, CCSD(T), in conjunction with the correlation-consistent cc-pVTZ basis set, and fitted by three analytical potential functions in terms of the Simons–Parr–Finlan (SPF) polynomial, Jensen function and the Sorbie–Murrell (SM) function. Ab initio single-point calculations over a distributed range of grids are performed first, and totally 12 085 converged points are fed into these functions. The fitting of each analytical PES function is done with an unconstrained minimization of the difference between the evaluations of the analytical function and the ab initio results, solved by a modified Levenberg–Marquardt algorithm with a finite-difference Jacobian in the IMSL package. The SPF polynomial is found to have the best global description, while the SM function behaves superior in the dissociation region forming three atomos. The spline function is potentially feasible to interpolate the computationally divergent points in the ab initio calculations.