Natural and Dyson orbitals in small helium drops

Abstract The natural and Dyson orbitals are studied for small helium drops comprising 5 to 20 helium atoms interacting via a soft two-body Gaussian potential. The wave functions of these drops have been obtained in the hyperspherical cluster model (HCM) which provides a correct description of the single-particle behavior at large separations from the system. The natural orbitals are obtained from diagonalization of the nonlocal one-body density matrix, while Dyson orbitals are constructed by the direct overlap of the wave functions of two drops differing by one boson. This overlap converges with an increasing basis of the HCM. The shapes and occupancies of the natural orbitals, as well as their link to Dyson overlap and evolution with increasing number of atoms, are discussed. Both natural and Dyson orbitals can be used to represent the density of the system. However, the natural orbital representation is demonstrated to be superior. With increasing boson numbers the difference between Dyson and natural orbitals becomes less prominent and is expected to disappear in infinitely large systems of identical bosons.