ABSTRACT
Coupled-cluster (CC) method has been established as a state-of-the-art technique in the calculation of electronic structure, spectra and properties. CC theory, due to the non-linear structure of the wave-operator, is computationally demanding. However, excellent progress in algorithms coupled with developments in computers, has made CC an affordable method for electronic structure calculation for small to medium size molecules. The initial development of the linear response formulation of CC theory used a non-variational method of projection, which involved the nth derivative of the cluster amplitudes in the computation of the nth order energy derivatives. While single reference based CC theories can be used away from equilibrium, a more desirable way to include the effects of quasi-degeneracy is to use an appropriate multi-determinantal starting space instead of Hartree-Fock single determinant. The use of relaxed orbitals improves the results significantly at large bond distances.
