Almost exact energies for the G1/G2 set with the semistochastic heat-bath configuration interaction method

The recently developed semistochastic heat-bath configuration interaction (SHCI) method is a systematically improvable selected configuration interaction plus perturbation theory method capable of giving essentially exact energies for larger systems than is possible with other such methods.We compute SHCI atomization energies for the 55 molecules in the G1/G2 set, for which accurate experimental data are available. Basis sets from cc-pVDZ to cc-pV5Z are used, totaling up to 500 orbitals and a Hilbert space of $10^53$ determinants for the largest molecules. To speed up convergence, we first optimize orbitals using 1-body and 2-body reduced density matrices constructed from SHCI wavefunctions with a large convergence threshold. For each of the basis sets, the extrapolated energy is within chemical accuracy (1 kcal/mol) of the exact energy for that basis using only a tiny fraction of the entire Hilbert space. The energies are extrapolated to the basis set limit and compared to the experimental atomization energies. We also use our almost exact energies to benchmark coupled cluster theory (CCSD(T)) energies.