Chapter 8 - Potential Energy Curves via Double Ionization Potential Calculations: Example of HF Molecule

Abstract

Recently developed coupled-cluster (CC) method (J. Chem. Phys., 135, 044121, 2011) to study doubly ionized systems has been applied to describe the ground and excited state potential energy curves (PECs) of the HF molecule. The method is based on a multi-reference CC scheme formulated within the Fock-space (FS) framework. The reason for using this kind of method in the calculation for the neutral system is that the ground state of HF–2 ion is well described by a single-reference approach for all internuclear distances while for the neutral HF this is not the case. The closed shell HF molecule dissociates into open shell subsystems while HF–2 gives closed shell fragments. Having reliable description of HF–2 and the FS-CC strategy the effect of removing two extra electrons can be efficiently calculated to provide good quality PECs for the neutral HF molecule for the whole range of internuclear distances up to the full separation. The FS-CC method allows the simultaneous description of the ground and excited states. This FS-CC strategy is confronted with the standard CC calculations for the ground state of HF followed by the Equation-of-Motion CC excitation energy calculation for excited states. The FS-CC approach is derived using the intermediate Hamiltonian formalism offering an intruder-free efficient computational scheme. The basic FS-CC approximation with the cluster operator restricted to one- and two-particle contributions is employed in the calculations. Our results for HF show that the FS-CC method is capable of providing smooth PECs for the whole range of internuclear distances from the equilibrium to the dissociation limit.