Using Quadrature and an Iterative Eigensolver to Compute Fine-Structure Ro-Vibrational Levels of Van der Waals Complexes: NH(Σ−3)–He, O2(Σg−3)–Ar, and O2(Σg−3)–He

Abstract

We introduce a new method for computing spectra of molecules for which a spin-spin term in the Hamiltonian has an important effect. In previous calculations, matrix elements of the spin-spin term and of the potential were obtained by expanding the potential and using analytic equations in terms of 3–j symbols. Instead, we use quadrature. Quadrature is simple and makes it possible to do calculations with a general potential and without using the Wigner-Eckart theorem. In previous calculations, the Hamiltonian matrix was built and diagonalized. Instead, we use an iterative eigensolver. It makes it easy to work with a large basis. The ideas are tested by computing energy levels of NH(3Σ−)–He, O2(Σg−3)–Ar, and O2(Σg−3)–He.