A modified Variable-Phase algorithm for multichannel scattering with long-range potentials

A new Variable-Phase (VP) algorithm for solving the close coupled equations of inelastic scattering in atom-molecule collisions driven by a strong long range potential is presented. The proposed method allows for a rigorous, gadual reduction of the number of closed channels during the outward propagat ion of the solution of the VP equations. In this way it allows a considerable saving of CPU time when dealing with strong, long-range potentials. A further saving of computational time is achieved by the use of a zero order effective potential in the reference problem which avoids the calculation of the compu tationally expensive Bessel functions. The K matrix version of the VP equations are solved with a standard RungeKutta integrator with adaptive step size. T he low-energy. rotational excitation process in the LiH-H+ system is used to test the resulting algorithm and we show that the present method once applied to long-range interactions. can be orders of magnitude faster than the widely used. adaptive-step size LoggDerivative/Airy propagator while keeping the sam e level of accura