Direct calculation of resonance energies and widths using anR-matrix approach

A modified $R$-matrix technique is presented which determines the eigenvalues and widths of resonant states by the direct diagonalization of a complex, non-Hermitian matrix. The method utilizes only real basis sets and requires a minimum of complex arithmetic. The method is applied to two problems, a set of coupled square wells and the ${\Pi }_g$ resonance of ${\mathrm{N}}_2$ in the static-exchange approximation. The results of the calculation are in good agreement with other methods and converge very quickly with basis-set size.