  For the Hirshfeld-I atom-in-molecule model, associated single-atom energies
and interaction energies at the Hartree-Fock level are determined efficiently
in one-electron Hilbert space. In contrast to most other approaches, the energy
terms are fully consistent with the partitioning of the underlying one-electron
density matrix. Starting from the Hirshfeld-I atom-in-molecule model for the
electron density, the molecular one-electron density matrix is partitioned with
a previously introduced double-atom scheme [Vanfleteren D. et al., J Chem Phys
2010, 132, 164111]. Single-atom density matrices are constructed from the
atomic and bond contributions of the double-atom scheme. Since the Hartree-Fock
energy can be expressed solely in terms of the one-electron density matrix, the
partitioning of the latter over the atoms in the molecule leads naturally to a
corresponding partitioning of the Hartree-Fock energy. When the size of the
molecule or the molecular basis set does not grow too large, the method shows
considerable computational advantages compared to other approaches that require
cumbersome numerical integration of the molecular energy integrals weighted by
atomic weight functions.
