Approximate ab initio calculations on polyatomic molecules

All-electron wavefunction calculations have been performed on the molecules H 2 O, CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , using both molecular orbital and group function methods. The calculations are essentially ab initio, but are greatly simplified by choosing the basis functions so as to reduce the many-centre two-electron integrals to values so small that they may be neglected without significant effect on the computed wave functions. Comparisons with exact minimal basis calculations confirm the validity of the approximations. It would appear that the method may be applied effectively and without undue difficulty to much larger molecules.