Convergence of the electronic density for a target region in cluster models of a NH$$_3$$ molecular crystal

Abstract

AbstractIn this paper we illustrate the advantage of addressing size-intensive properties of target regions without first converging the ground-state energy of that region. We use local occupied and virtual orbitals to separate the orbital space of NH$$_3$$ 3 clusters into an orbital space for the target region (a central NH$$_3$$ 3 molecule) and for the remaining cluster. Convergence characteristics of the Hartree–Fock (HF) energy and, indirectly, the electronic density of the target region are shown. The calculations illustrate that although the energy of the target region will not converge with cluster size, the electronic density will. The convergence of the electronic density of the target region is subsequently exploited to obtain HF dipole moments and CC2-in-HF vertical excitation energies. For these properties convergence is seen upon the inclusion of approximately three shells beyond the target region. This shows that local size-intensive properties of a target region can be investigated without converging the energy. We further show that a minimal basis description of the outer shells are sufficient to capture the correct interaction with the target region. The possibility of computing size-intensive properties for a target region using a converged electronic density, without requiring convergence in the energy itself, is currently an underexploited feature.