  A full six-dimensional Born-Oppenheimer singlet potential energy surface is
constructed for the reaction CaF + CaF $\rightarrow$ CaF$_2$ + Ca using a
multireference configuration interaction (MRCI) electronic structure
calculation. The {\it ab initio} data thus calculated are interpolated by
Gaussian process (GP) regression. The four-body potential energy surface
features one $D_{2h}$ global minimum and one $C_s$ local minimum, connected by
a barrierless transition state that lends insight to the reaction mechanism.
This surface is intended to be of use in understanding ultracold chemistry of
CaF molecules.
