  We construct $bb\bar{u}\bar{d}$ states on lattice using NRQCD action for
bottom and HISQ action for the light up/down quarks. The NRQCD-HISQ tetraquark
operators are constructed for "bound" $[bb][\bar{u}\bar{d}]$ and "molecular"
$[b\bar{u}] [b\bar{d}]$ states. Corresponding to these different operators, two
different appropriately tuned light quark masses are needed to obtain the
desired spectra. We explain this requirement of different $m_{u/d}$ in the
light of relativised quark model involving Hartree-Fock calculation. The mass
spectra of double bottom tetraquark states are obtained on MILC $N_f=2+1$
Asqtad lattices at three different lattice spacings. Variational analysis has
been carried out to obtain the relative contribution of "bound" and "molecular"
states to the energy eigenstates.
