The improved virtual orbital-complete active space configuration interaction method, a "packageable" efficient ab initio many-body method for describing electronically excited states

We describe a computationally efficient ab initio many-body method that can be used as a "packageable approximation" for computing excited state prope rties for small to large molecular systems, including those of multiconfigu rational character. The method is based on first order multi-reference many -body perturbation theory (MR-MBPT), where the unoccupied valence orbitals are obtained by using an extension of Huzinaga's improved virtual orbital ( IVO) generation technique. Because the method employs a complete active spa ce (CAS) which contains singly, doubly, and higher excited state configurat ions with respect to the zeroth order ground state configuration, the appro ach (IVO-CASCI) is capable of providing a more accurate description of the excited states than the widely used packageable configuration interaction w ith singles (CIS) at a fraction of computational labor. Moreover, unlike th e CASSCF approach this IVO-CASCI method does not require iterations and the refore is more computationally efficient and free of the convergence proble ms that sometimes plague CASSCF calculations with increasing size of the CA S. Excited state energies are compared with energies from the widely used C IS, MCSCF, and CASSCF methods for the C2H+, C2H, CaOH, cyclic-C3H, and porp hin molecules. The computed IVO-CASCI transition energies are generally mor e accurate than the CASSCF. For example, our energies are comparable to CIS energies for CaOH and porphin, while the C2H+, C2H, and C3H IVO-CASCI tran sition energies are more accurate than the CASSCF and CIS energies. (C) 200 1 American Institute of Physics.