State-Of-The-Art Computations of Vertical Electron Affinities with the Extended Koopmans’ Theorem Integrated with the CCSD(T) Method

Abstract

Accurate computation of electron affinities (EAs), within 0.10 eV, is one of the most challenging problems in modern computational quantum chemistry. The extended Koopmans' theorem (EKT) enables direct computations of electron affinities (EAs) from any level of the theory. In this research, the EKT approach based on the coupled-cluster singles and doubles with perturbative triples [CCSD(T)] method is applied to computations of EAs for the first time. For efficiency, the density-fitting (DF) technique is used for two-electron integrals. Further, the EKT-CCSD(T) method is applied to three test sets of atoms and closed-and open-shell molecules, denoted A16, C10, and O33, respectively, for comparison with the experimental electron affinities. For the A16, C10, and O33 sets, the EKT-CCSD(T) approach, along with the augcc-pV5Z basis set, provide mean absolute errors (MAEs) of 0.05, 0.08, and 0.09 eV, respectively. Hence, our results demonstrate that high-accuracy computations of EAs can be achieved with the EKT-CCSD(T) approach. Further, when the EKT-CCSD(T) approach is not computationally affordable, the EKT-MP2.5, EKT-LCCD, and EKT-CCSD methods can be considered, and their results are also reasonably accurate. The huge advantage of the EKT method for the computation of IPs is that it comes for free in an analytic gradient computation. Hence, one needs neither separate computations for neutral and ionic species, as in the case of common approaches, nor additional efforts to obtain IPs, as in the case of equation-of-motion approaches. Overall, we believe that the present research may open new avenues in EA computations.

Ab Initio Electron Propagators with an Hermitian, Intermediately Normalized Superoperator Metric Applied to Vertical Electron Affinities

“…The resulting 50 (NIST-50-EA) molecules are chemically diverse, main-group compounds that contain every element in the first three periods except He, Ne, and Al. This set represents an improvement over prior attempts ,, to create EA standards for small, closed-shell molecules. It also contains all small, closed-shell molecules present in a compendium of experimental and theoretical EAs.…”

Achieving Accuracy and Economy for Calculating Vertical Detachment Energies of Molecular Anions: A Model Chemistry Composite Methods

“…It is worth noting that the Koopmans’ theorem related to the above‐mentioned EPT methods is based on Hartree‐Fock (HF) or self‐consistent field (SCF) wave functions. By combining ionization with an orbital model based on multiconfigurational‐self‐consistent‐field (MCSCF) wave function, an extended Koopmans’ theorem (EKT) has also been developed [27–29] …”

“…By combining ionization with an orbital model based on multiconfigurational-self-consistent-field (MCSCF) wave function, an extended Koopmans' theorem (EKT) has also been developed. [27][28][29] How to conquer the contradiction between accuracy and economy in VDE prediction without any physical worry? We are aware that for predicting thermochemical properties (e. g., dissociation energies, ionization potentials, electron affinities, and proton affinities), a series of composite methods, i. e., Gn, [42][43][44][45] CBS, [46][47][48][49] and Wn, [50] have been well developed to compute molecular energies that can reach the chemical accuracy of 1-2 kcal/mol (i. e. 0.0434-0.0867 eV) or better for small and medium sized molecules.…”