The complete anharmonic cubic and quartic force field of the two carbonyl stretching vibrations of a rhodium di-carbonyl complex is calculated at the density functional level and used to simulate the third-order vibrational response function. The infrared photon echo spectrum calculated using the diagonalized resulting exciton Hamiltonian is in qualitative agreement with measured values. Quartic terms in the potential are critical for reproducing the experimental transition energies and transition dipoles. (C) 2003 American Institute of Physics.