Theory of perturbations is used in order to derive homogeneously the diatomic Hamiltonian starting from electronic structure up to the higher orders now necessitated in spectrum analysis, taking into account all the interactions without nuclear spins. A diagrammatic method is presented, enlightening the high orders complex calculations and the physical significance of the parameters. All the calculations are made for electronic states in Hund's coupling cases a, b, intermediate a–b and c tendency, and are suitable for modern numerical approaches. Effects up to fourth order (Λ-doubling in Δ states) are investigated. Application is made for a 4Λ (Λ ≥ 1) states.