A new global and compact analytical representation of the potential energy surface of triatomic systems: Application to copper nitrosyl and water

A new, general analytical representation of potential energy surfaces (PES) for triatomic molecules is developed. It is based on a novel, angular-dependent form of the Morse potential extended to two bonds. The very compact analytical form needs only a few adjustable parameters while yielding a physically sound description of the global behaviour of the interactions in the entire configuration space. As a first example, the global PES of the lowest adiabatic state of water monomer (H₂O) is reviewed. Parameters were adjusted to sets of energy points obtained from ab initio calculations at the coupled cluster and multi-reference configuration interaction level of theory. Experimental vibrational band centres of levels below 5000 cm^-1 are correctly reproduced to within 3 cm^-1; higher lying levels are less well described than in previously determined PES representations which involved a substantially extended set of adjustable parameters. Spectroscopic and thermodynamic data as well as reaction barriers are overall qualitatively well reproduced. The representation is suitable to describe semi-quantitatively the recombination (²S)H + (²Π)OH = (¹A ₁)H₂O as well as the abstraction reaction (²S)H + (²Π)OH = (¹Σ_g⁺)H ₂ + (³P)O in the limit of low collision energies. As a second example, the first global PES of the copper nitrosyl molecule (CuNO) is presented. The equilibrium structure with a bent Cu-N-O connectivity and the dissociation energy match the currently best known values from ab initio calculations (Krishna and Marquardt, J. Chem. Phys. 136, 244303 (2012)) and vibrational band centres agree well with previously published experimental data from matrix isolation spectroscopy for various isotopologues. A high lying metastable state with the linear connectivity N-Cu-O is predicted.