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 CH₂(³B₁, ¹A₁, ¹B₁ and ¹Σ(¹A)) and to the two lowest of CH(²A₁ and ²∏_u(²B₁)). 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 (C₂H₂, (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 CH₂ States are within 0.02 Å and 5° of available experimental data. The calculated term values and ionization potentials, T_e(¹A₁ = 0.35 eV, T_e (¹B₁) = 1.22 eV, T_e (¹Σ(¹A)) 2.48 eV, I.P. (²A₁) = 10.39 eV and I. P. (²∏_u(²B₁)) = 10.58 eV, are in agreement with some recent theoretical studies, and are very close to existing experimental information.