Extension of the active‐space equation‐of‐motion coupled‐cluster methods to radical systems: The EA‐EOMCCSDt and IP‐EOMCCSDt approaches

Abstract

Abstract The development of the active‐space variants of the electron‐attached (EA) and ionized (IP) equation‐of‐motion coupled‐cluster (EOMCC) methods, in which higher‐than‐doubly excited components of the cluster operator T and higher than 2 p ‐1 h or 2 h ‐1 p components of the electron attaching and ionizing operators R are defined through the use of active orbitals, is discussed. As shown by preliminary test calculations of adiabatic excitation energies and potential energy curves for the low‐lying states of the CH and OH radicals, the basic active‐space EA‐EOMCC and IP‐EOMCC methods, referred to as the EA‐EOMCCSDt and IP‐EOMCCSDt approaches, are capable of accuratelydescribing the ground and excited states of open‐shell systems, even at the moderately stretched nuclear geometries and even for states with a strong multi‐determinantal or doubly excited character, at the low costs that are on the order of those characterizing the standard EOMCC singles and doubles method. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006