Comparison between correlation crystal field calculations using extended basis sets and two-electron operators

There are two established methods for extending the crystal-field model for 4f(N) configurations. One is to add two-electron correlation crystal-field operators, the other is to extend the basis set to include more configurat ions. We consider the case of Pr3+ ions in LiYF4 and show, by projecting th e configuration-interaction Hamiltonian into the 4f(2) configuration, that in this case the two approaches give very similar results. (C) 2001 Elsevie r Science B.V. All rights reserved.