How to obtain accurate diabatic surfaces governing the dissociative recombination of astrophysical ions

Electronic dissociative recombination (DR) AB+ + e− → A + B processes are important for astrophysical, combustion and fusion plasmas. In the interstellar medium they determine the abundance of the positively charged species and knowing their efficiency is of primary importance. Their theoretical study requires two steps: (i) electronic structure calculations to obtain potential energy surfaces and corresponding coupling terms, and (ii) collision dynamic treatments using the potentials and the coupling terms obtained from the previous step, to calculate rates constants and branching ratio. The present work details the first step. The states involved in the DR of an ion are of different types, namely, the ionic, Rydberg and dissociating states of the corresponding neutral. Calculating them is not routine since their nature changes along the dissociating channels and carefully designed wavefunctions are needed to follow those transformations. Moreover, complications rise for the dissociating states that are embedded in the continuum of scattering states that correspond to AB+ + e-, because they are highly excited and strongly mixed with other states. In a DR process multiple curve crossings occur, and it can be very difficult to isolate the desired Rydberg and dissociating potential surfaces. In order to insure their quantitative treatment, crucial for accurate rate constants calculations though step (ii) we have developed over the past few years [1-3] a methodology that uses the block diagonalization method [4] to determine accurate diabatic surfaces from the MRCI adiabatic ones as well as the corresponding electronic couplings which are needed for the collision dynamic treatment (step ii). The power of this methodology is outlined through our study of the DR of HCNH+, N2H+ and SH+.