  We investigate the energy levels of heteronuclear alkali metal dimers in
levels correlating with the lowest rotational level of the ground electronic
state, which are important in efforts to produce ground-state ultracold
molecules. We use density-functional theory to calculate nuclear quadrupole and
magnetic coupling constants for RbK and RbCs and explore the hyperfine
structure in the presence of electric and magnetic fields. For nonrotating
states, the zero-field splittings are dominated by the electron-mediated part
of the nuclear spin-spin coupling. They are a few kHz for RbK isotopologs and a
few tens of kHz for RbCs isotopologs.
