  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($^3\Sigma^-$)-He, O$_2$($^3\Sigma^-_g$)-Ar and
O$_2$($^3\Sigma^-_g$)-He.
