Doubly contracted CI method

An approximate so-called doubly contracted CI method is proposed. In this method the contraction of configurations is performed not only over the external but also over the doubly occupied orbital space by the use of perturbation theory. Examples calculated based on the doubly contracted method show that the dimension of a CI space is drastically reduced and the computation time is reduced by an order of magnitude. The accuracy of the method is tested by calculating the multiplet splitting of vinylnitrene and potential curves of nitrogen molecule.