  We computed the long-range interactions between two identical polar bialkali
molecules in their rovibronic ground level, for all ten species involving Li,
Na, K, Rb and Cs, using accurate quantum chemistry results combined with
available spectroscopic data. Huge van der Waals interaction is found for eight
species in free space. The competition of the van der Waals interaction with
the dipole-dipole interaction induced by an external electric field parallel or
perpendicular to the intermolecular axis is investigated by varying the
electric field magnitude and the intermolecular distance. Our calculations
predict a regime with the mutual orientation of the two molecules but with no
preferential direction in the laboratory frame. A mechanism for the stimulated
one-photon radiative association of a pair of ultracold polar molecules into
ultracold tetramers is proposed, which would open the way towards the optical
manipulation of ultracold polyatomic molecules.
