The use of effective Hamiltonians for the treatment of avoided crossings. II. Nearly diabatic potential curves

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Abstract
For pt.I see ibid., vol.17, p.1235-57 (1984). The CI problem may be treated through the quasi-degenerate perturbation theory in order to avoid the delta psi / delta R nightmare which appears in the avoided-crossing region of adiabatic potential curves. A convenient modification of the CIPSI algorithm allows one to build a 2*2 (or p*p) effective Hamiltonian spanned by multiconfigurational wavefunctions, the diagonal energies of which represent non-adiabatic potential curves, coupled through a slowly varying effective interaction; the solution of this effective Hamiltonian gives accurate adiabatic potential curves and interpolations of the effective Hamiltonian matrix elements provide the switching function from a small number of points. In the former one, where the basic configurations differ by two spin orbitals (the crossing between 3p54s and 3p54p3 Pi g states of Ar2*), standard MO are used, while in the second case, where the two basic configurations differ by only one spin orbital (the crossing between the neutral and ionic configurations of NaCl), orthogonal atomic orbitals are used.