Incremental full configuration interaction

The incremental expansion provides a polynomial scaling method for computing electronic correlation energies. This article details a new algorithm and implementation for the incremental expansion of full configuration interaction (FCI), called iFCI. By dividing the problem into n-body interaction terms, accurate correlation energies can be recovered at low n in a highly parallel computation. Additionally, relatively low-cost approximations are possible in iFCI by solving for each incremental energy to within a specified threshold. Herein, systematic tests show that FCI-quality energies can be asymptotically reached for cases where dynamic correlation is dominant as well as where static correlation is vital. To further reduce computational costs and allow iFCI to reach larger systems, a select-CI approach (heat-bath CI) requiring two parameters is incorporated. Finally, iFCI provides the first estimate of FCI energies for hexatriene with a polarized double zeta basis set, which has 32 electrons correlated in 118 orbitals, corresponding to a FCI dimension of over 10³⁸.