Applications of size-consistent state-specific multi-reference coupled cluster (SS-MRCC) theory to study the potential energy curves of some interesting molecular systems

In this paper we apply the state-specific multi-reference coupled cluster method (SS-MRCC) (Mahapatra et al 1999 J. Chem. Phys. 110 6171) to compute the potential energy curves (PECs) of some interesting diatomic molecular systems. The theory is built on complete active space (CAS) reference functions. This formulation relies upon a small CAS, but is capable of producing energies to a high degree of accuracy. In the SS-MRCC method, depending upon the method of iteration, we may determine cluster amplitudes and consequently the effective Hamiltonian R either with the frozen (unrelaxed scheme) or with the converged coefficients (relaxed scheme) which are the model space coefficients in the final wavefunction. Thus the energy can be obtained either as an expectation value with respect to the unrelaxed function or by diagonalizing H within the CAS. The SS-MRCC method is rigorously size-extensive and size-consistent. The method bypasses the intruders by focusing on a single state of interest in a natural manner. The method could thus serve as a reliable choice for PEC studies over a wide range of geometries. We will also compare the relative performances of the description of 'unrelaxed' versus 'relaxed' coefficients. We also evaluate certain spectroscopic constants and compare them with experimental results.