Theoretical study of curve crossing: ab initio calculations on the four lowest 1Σ+ states of LiF

Abstract

Accurate ab initio potential energy curves for the classic Li-F ionic-covalent interaction by extrapolation to the complete basis set limit and modeling of the radial nonadiabatic couplingThe potential curves of the four lowest 1 ~ + states of LiF have been studied by ab initio configuration interaction methods, using a contracted Gaussian basis set of better than double-' quality, augmented with diffuse basis functions on the fluorine atom. The choice of orbitals and selection of configurations were carried out with the objective of obtaining a balanced treatment of the four states and of the different regions of the potential curves. "Generalized valence-bond" orbitals, obtained from ground-state MCSCF calculations, together with "improved virtual orbitals," were found suitable for the construction of the configuration functions. The latter were energy selected from all single and double excitations relative to a set of 12 "reference configurations," chosen to include all contributions found important for any of the four states at any internuclear distance. The expected avoided crossing between the lowest covalent and ionic structures is found at an internuclear distance of about 11 bohr, compared to the experimentally deduced value of 14 bohr, due to the considerable difficulty in obtaining a balanced description of the electron correlation of the separated species, particularly the F-ion. A set of diabatic potential curves was generated from the two lowest adiabatic curves by a novel procedure based on the Rittner model of the ionic state. A set of adjusted adiabatic potentials was obtained from the diabatic curves after shifting the ionic diabatic curve downward by 0.42 eV to correct for the error in the computed electron affinity of the fluorine atom, resulting in a shift of the avoided crossing to the correct internuclear distance of 14 bohr. The dipole moments of the four states were computed as a function of internuclear distance and were interpreted in terms of the electronic structure.culations on LiF, the Simplest of the alkali halide diatomics. Since there have been no rigorous calculations reported previously on these systems-except for ground states and often near the equilibrium geometry only4-6_ it has been the aim of this work to examine the full potential curves for a number of states, with emphasis on the nature of the avoided crossing.The ionic dissociation limit in LiF, Li+(lS) + F-(IS),

Multi-reference coupled-cluster study of the ionic-neutral curve crossing LiF

“…As already pointed out in the introduction the LiF molecule has been frequently studied in the literature by various groups [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The ionic and covalent states of 1 Σ + symmetry assemble an avoided crossing for an inter-atomic distance of ∼11.5 a.u.…”

“…Therefore, it has been frequently used in the literature as a benchmark system. Initially there were MCSCF and CI calculations of Kahn et al [1], Botter et al [2], and Werner et al [3]. Later Spiegelmann et al [4,5] applied effective Hamiltonian techniques.…”

Select-divide-and-conquer method for large-scale configuration interaction

“…1,[4][5][6][7][8][9][10] Within the CI approaches, highly correlated configuration interaction 11 ͑HCCI͒ methods, viz., CI going well beyond the singles and doubles treatment, constitutes a practical alternative to using a full Hamiltonian representation, particularly after the developments in the companion paper 12 to be referred to as I. One possibility is to select a priori a subspace S having invariant properties with respect to separate unitary transformations of the occupied orbitals and of distinct sets of correlation orbitals grouped in a well-defined manner, 11 giving rise to multireference CI ͑MRCI͒, [13][14][15][16][17][18] complete active space ͑CAS͒ CI, 19 restricted active space ͑RAS͒ CI, 20 and so on. 21 The corresponding CI-matrix eigenvalue problem is…”

QC-DMRG study of the ionic-neutral curve crossing of LiF

“…This system has been subject of various earlier theoretical studies [4,5,6]. Kahn, Hay and Shavitt [4] calculated the four lowest 1 Σ + states using the configurationinteraction (CI) methods. The dipole moment function for the lowest 1 Σ + states was studied by Werner and Meyer [6] using MCSCF wave functions.…”

“…The latter methods focus on dynamic correlation and are mandatory for achieving quantitative agreement with experiment. A frequently studied test system for the quantitative calculation of electron correlation effects in molecules is the ionic-neutral avoided crossing in the potential curves of alkaline halogenides, in particular of LiF [4,5,6,7,8,9,10,11,12]. The goal of these benchmark studies is the development of a computational protocol which provides a globally accurate description of the system even though the wave functions change dramatically transversing the avoided crossing.…”