The correlation contribution to the transition amplitudes are investigated in the case of Nd:LiYF4. By using many-body perturbation theory, we derive expressions for an effective dipole operator. The operators considered are Omega(es)((1)dagger)DOmega(cf)((1)), Omega(cf)((1)dagger)DOmega(es)((1)), Omega(es-cf)((2)dagger)D, and DOmega(es-cf)((2)). In contrast to third-order spin-orbit and crystal-field modified amplitudes, inclusion of correlation modifies the standard second-order amplitudes significantly. A model cluster is optimized to experimental energy levels. This approach is then used to compute consistent odd and even crystal-field parameters needed for theoretical absorption spectra. As expected, it is observed that transitions occurring at small wavelengths are quite heavily influenced by correlation. The overall agreement between experimental and theoretical spectra below 600 nm (above 17000 cm(-1)) is greatly improved when correlation is taken into account.