  Momentum-space approach to calculation of one-electron energies and wave
functions proposed initially by Fock for a hydrogen atom and considered later
by Shibuya, Wulfman, and Koga for diatomic molecules is applied to clusters
composed of three and more atoms. The corresponding basis set in the coordinate
space is of the Sturmian type since all the hydrogenlike orbitals in this set
have a common exponent, i.e., correspond to the same energy (as opposed to
one-electron atomic orbitals). By the examples of He$_4^{+7}$ and He$_6^{+11}$
cluster ions it is shown that increase in the number of orbitals in the set
results in rapid convergence of eigenenergies and eigenfunctions of highly
excited states. The momentum-space approach to the one-electron many-center
problem may be used for various solid-state and quantum-chemical applications.
