Representations of atomic virtual orbitals for use in semi-empirical molecular orbital calculations

In semi-empirical molecular orbital calculations it is frequently desirable to include in the basis set of atomic orbitals low energy virtual atomic orbitals. Criteria for representing the spectroscopically observed virtual orbitals by single Slater-type orbitals (STO) are discussed. Representations for atoms through polonium (excluding the lanthanide series) are proposed.