No abstract

In order to circumvent the problem of spin contamination in unrestricted Hartree-Fock based coupled cluster (CC) calculations, we present a new method of calculation for certain classes of open-shell systems. The approach ensures that the proper spin component of the resulting correlated wave function is projected out in the energy evaluation by the use of a reference function constructed from suitably chosen restricted open-shell Hartree-Fock or other orbitals. This single-reference open-shell spin-restricted CC method is applied to the calculation of ionization potentials in the N2 molecule, and it is shown that highly accurate results can be obtained in a 5s4pld basis. The mean error for all the principal ionization potentials of N2 compared to experiment is 0.45%.

A multireference coupled-cluster (MRCC) formulation for the direct calculation of excitation energies and ionization potentials is presented. The reference space connects a set of p–h excited determinants built from all the set of active particles and holes in the model space. This model space is incomplete, requiring a Fock-space approach and the postulate of a ‘‘universal’’ wave operator to arrive at a linked diagram expression for the effective Hamiltonian Heff, whose eigenvalues are the excitation energies for the problem. Use of a normal-ordered exponential cluster ansatz allows one to construct, hierarchically, the CC equations for the p–h model space starting from the ground state. We present an extension of an earlier formulation for excitation energies that allows us to have both active and inactive particles and holes in our method. Numerical applications are reported for the prototypical small molecules CO and N2.