Here T is the kinetic energy operator for the nuclei and V is the potential energy surface which is dependent on the nuclear geometry. The form of the kinetic energy operator depends on the choice of coordinates and is not the focus of this work. We will assume some set of coordinates has been chosen and the kinetic energy operator can be obtained exactly or at least sufficiently accurate within these coordinates. We will instead focus on the construction of the PES by fitting to electronic structure single point calculations.