Fast Heat-Bath Configuration Interaction*

For many quantum mechanical systems the entire Hilbert space is enormous, but the important part is many orders of magnitude smaller. We have recently developed the semistochastic heat-bath configuration interaction (SHCI) method for efficiently calculating such systems. It is a systematically improvable selected configuration interaction plus perturbation theory method that is capable of giving essentially exact energies for larger systems than is possible with other such methods. I present recent advances we have made to the method that allow us to use 2 billion variational determinants and trillions of perturbative determinants. Details about the key data structures and the parallelization are presented. We use the algorithm to compute the potential energy surface of the very challenging chromium dimer in the cc-pVDZ-DK basis, correlating 28 valence and the semicore electrons. The Hilbert space has 5*10²⁹ determinants. At equilibrium our energy agrees with the recent p-DMRG energy, but is more precise. We also present results for the homogeneous electron gas.