Mutual neutralization in collisions of Li + and F  S.M. Nkambule, P. Nurzia, Å. Larson ⇑ Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden article info Article history: Available online 14 August 2015 Keywords: Mutual neutralization Non-adiabatic Configuration interaction LiF abstract Mutual neutralization in collisions of Li + and F  is driven by an avoided crossing between the two lowest 1 R þ electronic states of the LiF system. These electronic states are computed using the multi-reference configuration interaction method. We investigate how the adiabatic potential energy curves and the non-adiabatic coupling element depend on the choice of the reference configurations as well as the basis set. Using diabatic states, the total and differential cross sections for mutual neutralization are computed. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Diatomic molecules of alkali halides possess avoided crossings between lower lying adiabatic states due to mixing between ionic and covalent configurations. These systems provide illustrative examples of the Born–Oppenheimer break-down and they have been the subjects of numerous theoretical studies [1–3]. One such system is the LiF molecule, where the ionic state crosses the cova- lent state associated with the ground state fragments, Li( 2 S)+F( 2 P), at an internuclear distance, R x of 13.7 a 0 . Just below the ion-pair threshold, there is another covalent limit, Li( 2 P)+F( 2 P), with a very large crossing distance (around 190 a 0 ) and hence the correspond- ing electronic coupling can be neglected [4]. Asymptotically, the ion-pair potential can be described by the Rittner potential, V ip R ð Þ¼ E th  1 R  aðLi þ ÞþaðF  Þ 2R 4 , with the polarizabilities given by aðLi þ Þ¼ 0:193 a 3 0 and aðF  Þ¼ 13:5a 3 0 [1]. The avoided crossing between the two lowest 1 R þ states of LiF have been extensively studied by ab initio methods. One of the first studies was performed in 1974 by Kahn et al. [5], using a multi-ref- erence configuration interaction (MRCI) approach, where single and double external excitations were included from a 12-configu- ration reference wave function. The calculated adiabatic states were diabatized using an orthogonal two-by-two transformation and by assuming the ion-pair state has the above mentioned Rit- tner form. The calculated crossing distance was found to be about 11.3 a 0 . In 1981, Werner and Meyer [1] performed state-averaged multi-configuration self-consistent field (MCSCF) calculation on the two lowest 1 R þ states of LiF. The states were diabatized by performing an orthogonal transformation of the adiabatic dipole matrix into a diagonal diabatic one. This approach gave a crossing distance of 13.3 a 0 . This was followed by a theoretical study of Bauschlicher and Langhoff [6], where MRCI calculation using state-averaged complete active space self-consistent field (CASSCF) orbitals were compared with configuration interaction calculations where the lowest three molecular orbitals [F(1s), Li(1s) and F(2s)] were kept doubly occupied, and all excitations among the remain- ing valence electrons were included. They found a curve crossing distance of 12.6 a 0 . This is significantly shorter than the one expected due to the use of a small basis set. Again the states were diabatized by assuming a Rittner form of the diabatic ion-pair state. The ion-pair potential energy curve was shifted in energy to obtain a correct crossing distance. The fourfold way methodol- ogy for direct diabatization [7] has been illustrated for the LiF sys- tem using multiconfiguration quasidegenerate perturbation theory calculations and a crossing distance of 12.5 a 0 was obtained [8]. To mention some of the more recent quantum chemistry calcu- lations on the LiF system, we have an extensive study by Varandas [9] in 2009 using the MRCI technique with a sequence of correla- tion consistent basis sets. The calculated energies were extrapo- lated to the complete one-electron basis set limit. The computed non-adiabatic coupling element was fitted to analytical forms and the obtained parameters were used to perform an adiabatic to diabatic transformation [9,10]. The extrapolated crossing dis- tance was found to be 13.66 a 0 , which is in very good agreement with the predicted one. The explicitly correlated multi-reference configuration interaction method, MRCI-F12, has been tested on the LiF system [11] using the same set of basis sets as the ones used by Varandas. It was demonstrated that with a given basis set and active space, the calculated curve crossing distance get signifi- cantly closer to the one expected when the MRCI-F12 scheme is used. http://dx.doi.org/10.1016/j.chemphys.2015.08.006 0301-0104/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: aasal@fysik.su.se (Å. Larson). Chemical Physics 462 (2015) 23–27 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys