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Abstract:
Using the Z-vector formalism the analytical gradient of the energy in the half-electron open-shell treatment is derived and implemented for semiempirical MNDO-type methods. The computation time is shown to scale asO(N3) with the size of the system, with the memory requirements growing asO(N2). The evaluation of the analytical gradient is significantly faster than the half-electron SCF calculation, so that routine full geometry optimizations become possible for large open-shell systems. The approach can easily be extended to the treatment of the small CI expansions typically encountered in semiempirical computations.
