Abstract
Cubic and quartic internal coordinate force constants are often obtained by a least-squares fit of a set of ab initio energies to an expansion of the potential energy. The reliability of such a procedure can now be determined, since analytic Cartesian cubic force constants are available which, when used in a finite difference procedure, also yield ab initio Cartesian quartic force constants. It is shown that unless the points of the energy grid are situated close to the equilibrium point unreliable results are obtained. This is why ab initio SCF anharmonic constants are more accurate than previously believed.
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