State-of-the-Art Computations of Vertical Ionization Potentials with the Extended Koopmans’ Theorem Integrated with the CCSD(T) Method

Abstract

The accurate computation of ionization potentials (IPs), within 0.10 eV, is one of the most challenging problems in modern computational chemistry. The extended Koopmans' theorem (EKT) provides a systematic direct approach to compute IPs from any level of theory. In this study, the EKT approach is integrated with the coupled-cluster singles and doubles with perturbative triples [CCSD(T)] method for the first time. For efficiency, the density-fitting (DF) approximation is employed for electron repulsion integrals. Further, the EKT-CCSD(T) method is applied to a set of 23 molecules, denoted as IP23, for comparison with the experimental ionization potentials. For the IP23 set, the EKT-CCSD(T) method, along with the aug-cc-pV5Z basis set, provides a mean absolute error of 0.05 eV. Hence, our results demonstrate that direct computations of IPs at high-accuracy levels can be achieved with the EKT-CCSD(T) method. We believe that the present study may open new avenues in IP computations.

PSI4 1.4: Open-source software for high-throughput quantum chemistry

“…Thanks to this new development, the entire SAPT0 level of theory (but not higher levels such as second-order, SAPT2) is now available in Psi4 without the single-exchange approximation. Preliminary numerical tests show [157][158][159] that the replacement of E (20) exch−disp (S 2 ) by its nonapproximated counterpart introduces inconsequential changes to the SAPT0 interaction potentials at short intermolecular separations. In contrast, the full E range, especially for interactions involving ions.…”

“…Electron affinities and ionization potentials can now be computed through second-order electron-propagator theory (EP2) 17 and the extended Koopmans' theorem (EKT). [18][19][20] Coupled-Cluster Theory. Psi4 supports conventional CC energies up to singles and doubles (CCSD) plus perturbative triples [i.e., CCSD(T)] 4 for any single determinant reference and analytic gradients for RHF and UHF references.…”

“…Psi4 v1.4 uses the -D3 correction in a new method, SAPT0-D. While SAPT0 has long been applicable to systems with upwards of 300 heavy atoms by leveraging optimized DF routines for both JK builds and MP2-like E (20) disp and E (20) exch−disp terms, it is limited by the O(N 5 ) scaling of the second-order dispersion (N proportional to system size). By refitting the -D3 damping parameters against a training set of nearly 2300 CCSD(T)/CBS IE and using the result in place of the analytic SAPT0 dispersion component, SAPT0-D at O(N 4 ) scaling achieves a 2.5× speedup for systems with about 300 atoms (increasing for larger systems), while removing the worst outliers and achieving the same error statistics as full SAPT0 when evaluated on 8100 IE of bimolecular complexes.…”

“…The SAPT module in Psi4 now has an option to compute the second-order SAPT0 exchange corrections E (20) exch−ind,resp and E (20) exch−disp without the use of the common S 2 approximation, that is, using the complete antisymmetrizer in the expressions instead of its approximation by intermolecular exchanges of a single electron pair. The working equations for the non-approximate second-order corrections were derived and implemented for the first time in Refs.…”

“…157 and 158 in the molecularorbital (MO) form prevalent in the classic SAPT developments. We have recast the nonapproximate formulas for E (20) exch−ind,resp and E (20) exch−disp of Refs. 157,158 into the AO form and implemented them efficiently in Psi4 with DF.…”