17 July 1998 Ž . Chemical Physics Letters 291 1998 346–350 On the accuracy of reactive scattering calculations with absorbing potentials: a new implementation based on a generalized R-matrix propagation Fermın Huarte-Larranaga a , Xavier Gimenez a , Antonio Aguilar a, ) , Michael Baer b ´ ˜ ´ a Departament de Quımica Fısica, UniÕersitat de Barcelona, Martı i Franques, 1, 08028 Barcelona, Spain ´ ´ ´ ` b Department of Physics and Applied Mathematics, Soreq NRC, RehoÕot, Israel Received 8 April 1998; in final form 20 May 1998 Abstract A quantum scattering method based on combining a generalization of the propagative R-matrix technique with negative imaginary potentials is presented. Reactive probabilities are then obtained considering only the reactants arrangement channel and Jacobi coordinates. Collinear and infinite order sudden results are shown for the Cl qHCl symmetric reaction, showing excellent agreement with previous results, including the reproduction of sharp reactive scattering resonances, at a fraction of the computer time and memory requirements. q 1998 Elsevier Science B.V. All rights reserved. 1. Introduction Among the algebraic, numerical and computa- tional developments in molecular reactive scattering theory, special impetus has been taken recently in the use of complex absorbing potentials to decouple a w x subset of the close-coupling equations 1,2 . This began when Neuhauser and Baer realized that decou- pling the rearrangement channels could be achieved by simply adding a purely negative imaginary linear Ž . potential NIP ramp at the entrance of the products w x arrangement 3–5 . This idea exploited the fact that, for reactive collisions, the probability flux loss to- wards rearrangement states takes place in a rather ) Corresponding author. well-delimited region of configuration space, which is different from that where the inelastic flow trans- fer processes take place, between states of the same rearrangement channel. Therefore, a suitably defined function, depending on physical coordinates, is found to be able to discriminate the reactive component of the wavefunction and absorb it. This results in a local, smooth and weakly energy-dependent NIP. On such basis, powerful state-to-all time-depen- Ž . Ž . dent TD and time-independent TI methods for reactive scattering were developed, which avoided the well-known problem of artificial back-reflection Ž . of the wavepacket at the grid limits in TD methods and the coordinate transformation between rear- rangement channels. The combination of the TI ap- proach with a variational technique, applied to solv- ing the scattering problem by a splitting of the 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 98 00599-5