  The complex electronic structure and unusual potential energy curve of the
chromium dimer have fascinated scientists for decades, with agreement between
theory and experiment so far elusive. Here, we present a new ab initio
simulation of the potential energy curve and vibrational spectrum that
significantly improves on all earlier estimates. Our data support a shift in
earlier experimental assignments of a cluster of vibrational frequencies by one
quantum number. The new vibrational assignment yields an experimentally derived
potential energy curve in quantitative agreement with theory across all bond
lengths and across all measured frequencies. By solving this long-standing
problem, our results raise the possibility of quantitative quantum chemical
modeling of transition metal clusters with spectroscopic accuracy.
