Using nondirect product Wigner <i>D</i> basis functions and the symmetry-adapted Lanczos algorithm to compute the ro-vibrational spectrum of CH₄–H₂O

By doing calculations on the methane–water van der Waals complex, we demonstrate that highly converged energy levels and wavefunctions can be obtained using Wigner D basis functions Symmetry-Adapted Lanczos (SAL) method. The is a nondirect product and, therefore, efficient when kinetic operator has accessible singularities. SAL method makes it possible to exploit symmetry label reduce cost of calculation, without explicitly symmetry-adapted functions. Line strengths are computed, new bands identified. In particular, find unusually strong transitions between states associated with isomers global minimum secondary minimum.