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Structure and thermochemistry of K$_2$Rb, KRb$_2$ and K$_2$Rb$_2$

The formation and interaction of ultracold polar molecules is a topic of active research. Understanding possible reaction paths and molecular combinations requires accurate studies of the fragment and product energetics. We have calculated accurate gradient optimized ground state structures and zero point corrected atomization energies for the trimers and tetramers formed by the reaction of KRb with KRb and corresponding isolated atoms. The K${}_2$Rb andKRb${}_2$ trimers are found to have global minima at the $C_{2v}$ configurationwith atomization energies of 6065 and 5931 cm${}^{-1}$ while the tetramer is found to have two stable planar structures, of $D_{2h}$ and $C_s$ symmetry, which have atomization energies of 11131 cm${}^{-1}$ an 11133 cm${}^{-1}$, respectively. We have calculated the minimum energy reaction path for the reaction KRb+KRb to K$_2+$Rb$_2$ and found it to be barrierless.