Rho-axis-method Hamiltonian for molecules having one methyl rotor and C1 point-group symmetry at equilibrium

Two modifications of the rho-axis-method torsion-rotation Hamiltonian for fitting spectra of methyl top molecules belonging to the Cs point group at equilibrium which are required to treat molecules with C1 equilibrium configurations are described: (i) The permutation-inversion group must be changed from G6 to G3, which causes the A1 and A2 species to coalesce into a single A species, and which also introduces the slight complication of separably degenerate E species. (ii) One term must be added to the second-order contact-transformation-reduced Hamiltonian and seven terms must be added to the fourth-order reduced Hamiltonian. Programming strategies for this Hamiltonian are also discussed. In particular, portions of the C1 computer code requiring complex arithmetic are indicated, and three schemes for checking the internal consistency of the C1 code are suggested. Application of this C1 Hamiltonian to ethylacetamidoacetate is given in the preceding paper.