Size extensivity of a general-model-space state-universal coupled-cluster method

We show that the recently formulated general-model-space (GMS), state-universal (SU), coupled-cluster (CC) method, exploiting the so-called C-conditions, is size extensive (in the sense that the energy of a composite system consisting of noninteracting subsystems equals the sum of the subsystem energies, while employing the appropriate method in each case). This assumes that the reference or model space employed for the composite system is size consistent, i.e., is given by a tensor product of subsystem model spaces. For pedagogical reasons, the demonstration is carried out for the single reference case before the multireference SU CC case is addressed. The importance of the C-conditions and a relationship with earlier approaches is discussed, as well as the limiting case of a complete-model-space SU CC method. Finally, the size extensivity of a GMS-based SU CCSD method is illustrated by actual numerical examples. (C) 2004 Wiley Periodicals, Inc.