The effects of optimization and scaling of AO exponents on molecular properties

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The combined effects of direct optimization and various scaling techniques of Gaussian AO basis sets on the calculated molecular wavefunctions were analyzed for two examples, the hydrogen fluoride and ammonia molecules. An 8s4p Gaussian basis set for the fluorine atom which had been rigorously optimized for the total atomic energy by the conjugate gradient method was employed. For nitrogen, a similar atom optimized 9s5p basis set was employed. Three basis sets obtained in the course of the exponent optimization, the final optimized basis set, and basis sets derived from the optimum AO basis sets by various scalings of the exponents were employed in molecular calculations. The total energies and numerous one-electron properties were analyzed. The convergence of molecular one-electron properties to their limiting values is much slower than the energy convergence, and basis sets optimized for energy may be improved in terms of their property predictions by rigorous optimization. The simple scaling procedure slightly improved the total molecular energy but not necessarily the one-electron properties relative to the molecular calculation which employed the fully optimized atomic bases. Though it was generally assumed that with proper scaling of optimum AO basis sets, significant improvement can be achieved in a molecular wavefunction, the generality of this assumption is brought into question by the present examples of the hydrogen fluoride and ammonia molecules. Basis sets optimized directly by the conjugate gradient method for atoms appear to be a good compromise if the overall quality of the molecular basis is of interest and not just the total energy.