1. R. Gáspár and I. Tamássy-Lentei, Acta Phys. Hung.,33, 387, 1973;38, 15, 1975;40, 283, 1976;44, 191, 1978;50, 343, 1981.
2. I. Tamássy-Lentei and Á. Derecskei-Kovács, Acta Phys. Chim. Debr.,23, 137, 1981; J. Mol. Struct. (Theochem.),167, 217, 1988; Int. J. Quant. Chem.,36, 277, 1989.
3. H. Hellmann, Acta Physicochim. USSR,1, 913, 1935;4, 225, 1936; J. Chem. Phys.,3, 61, 1936.
4. K. Ladányi, Acta Phys. Hung.,5, 361, 1956.
5. H. Hylleraas, Z. Phys.,48, 469, 1928;54, 347, 1929.