  In this work we demonstrate that the heat bath configuration interaction
(HCI) and its semistochastic extension can be used to treat relativistic
effects and electron correlation on an equal footing in large active spaces to
calculate the low energy spectrum of several systems including halogens group
atoms (F, Cl, Br, I), coinage atoms (Cu, Au) and the Neptunyl(VI) dioxide
radical. This work demonstrates that despite a significant increase in the size
of the Hilbert space due to spin symmetry breaking by the spin-orbit coupling
terms, HCI retains the ability to discard large parts of the low importance
Hilbert space to deliver converged absolute and relative energies. For
instance, by using just over $10^7$ determinants we get converged excitation
energies for Au atom in an active space containing (150o,25e) which has over
$10^{30}$ determinants. We also investigate the accuracy of five different
two-component relativistic Hamiltonians in which different levels of
approximations are made in deriving the one-electron and two-electrons
Hamiltonians, ranging from Breit-Pauli (BP) to various flavors of exact
two-component (X2C) theory. The relative accuracy of the different Hamiltonians
are compared on systems that range in atomic number from first row atoms to
actinides.
