A critical comparison of theoretical and experimental electronic spectrum and potential energy curves of HF molecule and its positive and negative ions

Although ab initio many-body methods can provide accurate predictions for m ost small systems, substantial effort is devoted to design and develop fair ly accurate and computationally efficient 'packageable' methods for treatin g large molecular systems, where packageable implies a method that can be i ncorporated into widely distributed suites of electronic structure programs . Here, we provide further tests and illustrations of our recently develope d improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method, which is designed to supplant CIS and CASSCF approaches in electronic structure 'packages' because of greater computational effici ency without sacrificing accuracy. The IVO-CASCI method is used here to com pute the ground, excited, positive, and negative ion potential energy curve of hydrogen fluoride. Additional dynamical correlation is incorporated by using IVO-CASCI wavefunctions as the initial approximation in selected furt her effective valence shell Hamiltonian computations for BF and its ions. T he excellent agreement between the theory, experiment, and some benchmark c alculations for several ground, excited, and ion states of HF reinforces th e claim that the IVO-CASCI method is a viable alternate packageable many-bo dy method for ground, excited, and other open-shell states of small to larg e molecular systems. (C) 2001 Elsevier Science B.V. All rights reserved.