Simplified methods for ab initio calculations. The valence states of CH₂ and CH

Three simplifying methodds are discussed and applied to the four lowest valence states of CH 2 ( 3 B 1 , 1 A 1 , 1 B 1 and 1 Σ ( 1 A )) and to the two lowest of CH ( 2 A 1 and 2 ∏ u ( 2 B 1 )). These methods concern: (1) the development of polarization functions for Gaussian‐lobe basis sets by least‐square fitting of numebrical multiconfigurational atomic fuinctions (this approach is tested also on (C 2 H 2 , (2) the use of intermediate Hamiltonians to calculate avoided crossings between potential hypersurfaces, and (3) thecalculation of correlation energies using an atoms‐in‐molecule approach. The calculated equilibrium geometries of the CH 2 States are within 0.02 Å and 5° of available experimental data. The calculated term values and ionization potentials, T e ( 1 A 1 = 0.35 eV, T e ( 1 B 1 ) = 1.22 eV, T e ( 1 Σ ( 1 A )) 2.48 eV, I.P. ( 2 A 1 ) = 10.39 eV and I. P. ( 2 ∏ u ( 2 B 1 )) = 10.58 eV, are in agreement with some recent theoretical studies, and are very close to existing experimental information.