The impact of higher polarization basis functions on molecular ab initio results. I. The ground state of F2

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Abstract
The F2 ground state potential curve has been computed near re by means of the MCSCF, CI(SD), MR‐CI(SD) and the recently proposed CPF method. Extensive sets of polarization functions including up to five d sets, three f sets, and one g set have been used to study the convergence of the total energy of De, and re for these methods on basis set extension. SCF and MCSCF show a similar behavior and are practically converged on the 2d1f level. Convergence is much slower for methods which include external correlation, such as CI(SD), MR‐CI(SD), CPF. The largest calculation performed—MR‐CI(SD) for a [6 4 2 1] basis—included 280 076 configuration state functions, and yields De and re with an accuracy of 0.07 eV and 0.015 a.u. These remaining errors are attributed mainly to the neglect of g functions. The present results provide detailed information concerning the relative importance of polarization functions which can be ordered as: d(1)>d(2)≳f(1)>d(3) for SCF and MCSCF and d(1)>f(1)≳d(2)>g(1)≳f(2)≳d(3) for methods including external correlation.