The generation of continuum orbitals for molecular R-matrix calculations using Lagrange orthogonalisation

A method is presented whereby a given set of single-channel continuum functions can be used to generate a set of multi-channel orbitals which are orthogonal to a specified number of target molecular orbitals with the same symmetry. The procedure uses the single-channel functions as a basis set expansion which is Lagrange orthogonalised to the target orbitals by searching for the zeros of a determinant. Sample calculations on a variety of diatomic targets, such as H2+ and CO, show the method is reliable and does not suffer from the linear dependence problems frequently encountered with other orthogonalisation procedures. A straightforward generalisation of the method allows continuum orbitals to be generated which are adapted to anisotropic terms in the potential. (orig.)