Direct determination of long-range inverse-power potential coefficients from spectroscopic data

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Abstract
This paper presents and tests a method for determining the set of inverse‐power coefficients defining the long‐range potential tail of a given molecular state by least‐squares fitting directly to the energies and inertial rotational constants (Bv ’s) of vibrational levels lying very near dissociation. This approach is superior to the widely used procedure of determining potential points by some inversion procedure and fitting them to the inverse‐power form, in that the fit is directly sensitive to the uncertainties in the experimental quantities, and that the parameter uncertainties due to interparameter correlation are greatly reduced. Moreover, the new method gives direct estimates of the number, energies, and other properties of unobserved levels lying above those being fitted. Improved results, including potential parameters up to C10, are presented for the B(3Π+0u) state of I2.