Towards a general multireference coupled cluster method: automated implementation of open-shell CCSD method for doublet states

We present the outline for a general internally contracted multireference coupled cluster method, and indicate some open ended issues that are not yet resolved. The detailed equations for this general scheme are rather complicated and it is necessary to automate the implementation of the method. In this paper, we investigate the simplest variant of this scheme, a spin-adapted open-shell CCSD method applicable to open-shell doublet states. The method was implemented in an automated fashion, and we consider a number of slight variations on the general theme. Applications are presented for the cationic states of N-2, C2H4, C-2 and O-3, and anionic states Of C-2, O-3 and F-2, which include various highly correlated states that illustrate the potential of the general methodology. (C) 2001 Elsevier Science B.V. All rights reserved.