Adaptive multiconfigurational wave functions

A method is suggested to build simple multiconfigurational wave functions specified uniquely by an energy cutoff A. These are constructed from a model space containing determinants with energy relative to that of the most stable determinant no greater than A. The resulting A-CI wave function is adaptive, being able to represent both single-reference and multireference electronic states. We also consider a more compact wave function parameterization (A + SD- CI), which is based on a small A-CI reference and adds a selection of all the singly and doubly excited determinants generated from it. We report two heuristic algorithms to build A-CI wave functions. The first is based on an approximate prescreening of the full configuration interaction space, while the second performs a breadth-first search coupled with pruning. The A-CI and A+SD-CI approaches are used to compute the dissociation curve of N-2 and the potential energy curves for the first three singlet states of C-2. Special attention is paid to the issue of energy discontinuities caused by changes in the size of the A-CI wave function along the potential energy curve. This problem is shown to be solvable by smoothing the matrix elements of the Hamiltonian. Our last example, involving the Cu2O22+ core, illustrates an alternative use of the A-CI method: as a tool to both estimate the multireference character of a wave function and to create a compact model space to be used in subsequent high-level multireference coupled cluster computations. (C) 2014 AIP Publishing LLC.