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 properties for small to large molecular systems, including those of multiconfigurational 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 space (CAS) which contains singly, doubly, and higher excited state configurations with respect to the zeroth order ground state configuration, the approach (IVO-CASCI) is capable of providing a more accurate description of the excited states than the widely used packageable configuration interaction with singles (CIS) at a fraction of computational labor. Moreover, unlike the CASSCF approach this IVO-CASCI method does not require iterations and therefore is more computationally efficient and free of the convergence problems that sometimes plague CASSCF calculations with increasing size of the CAS. Excited state energies are compared with energies from the widely used CIS, MCSCF, and CASSCF methods for the C2H+, C2H, CaOH, cyclic-C3H, and porphin molecules. The computed IVO-CASCI transition energies are generally more 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 transition energies are more accurate than the CASSCF and CIS energies. (C) 2001 American Institute of Physics.