A new algorithm for Hamiltonian matrix construction in electron-molecule collision calculations

A method is presented for constructing configuration-interaction (CI) Hamiltonians for electron-molecule collision calculations based on close-coupling expansions. This method utilizes the structure of the wavefunction used in scattering calculations to greatly reduce the computational cost compared to conventional procedures. In particular, CI target configurations are treated only at the prototype configuration state function stage of the calculation. Only then are these prototype configurations expanded over all scattering functions using an improved version of a previous algorithm by Morgan and Tennyson. This new algorithm has been implemented as part of the UK R-matrix codes. Test calculations show that it leads to a large reduction in both the number of integrals and matrix elements which need to be explicitly constructed and evaluated. Use of this new program should allow significantly more sophisticated target wavefunctions to be employed in future scattering calculations.