A perturbative approach to the almost-linear coupled-cluster formalism

A very simple method for obtaining nearly size-extensive approximate solutions to the many-electron Schrِdinger equation is proposed and implemented. The method is based on the split-amplitude strategy within the framework of the coupled cluster (CC) methodology and amounts to solving the equations of the almost-linear CC (AL-CC) methods perturbatively. The new approach is referred to as a perturbative approach to the AL-CC equations (PA/AL-CC). Test calculations for the PA/AL-CCD method are performed for various model systems. Particular attention is paid to the performance of this method in the description of molecular states with diverse and varying degrees of quasidegeneracy.