Potential energy surfaces of charge transfer states

Abstract

In this paper the potential energy curves of CT states and their interaction with local ones have been investigated. Besides the global view of these curves, special attention has been paid to the region of the crossing and the infinite separation limit. It was found that triple excitations are needed to accurately describe potential energy surfaces of CT states. Among the cheaper variants, both STEOM-CCSD and CCSD(T)(a)* methods are promising in this respect. The somewhat larger error of CCSD for CT states can be explained by its size extensivity error and the overestimation of the asymptotic excitation energy. Second order approximations are not advantageous for the error cancellation, in fact CC2 is much worse for CT states than any other method investigated here. The results also show that the location of the (avoided) crossings of local and CT states depend very much on the accurate description of the CT states. Failure to describe this topology might affect dynamics, and a warning, in particular in case of CC2, should be issued if CT states play a role in the physics of the problem.

Assessing challenging intra‐ and inter‐molecular charge‐transfer excitations energies with double‐hybrid density functionals

“…∞) still lies below a local excited-state, which is not what it is predicted by CC3 and higher-order methods. 104 The same behavior is expected to be characteristic of other range-separated DH functionals too, further proving that there still is some margin of improvement available for these methods.…”

“…We have chosen the ammonia-fluorine complex, see Figure 3, for which nearly-exact CCSDT-3/cc-pVDZ results are available in both situations (6.62 eV at R = R e and 10.82 eV at R ! ∞, see Table S4) as reference values, 90,104 and checked that the excitation energy at R ! ∞ is always characterized by the CT descriptors Ω POS = 1.5 and Ω CT = 1.0 (see Table S5).…”

“…Actually, the latter seems to behave better than PBE-QIDH for all intermolecular distances, resembling closely the curves obtained using the CC2 method. 104 However, the RSX-QIDH limiting value of the CT state (at R ! ∞) still lies below a local excited-state, which is not what it is predicted by CC3 and higher-order methods.…”

A New Benchmark Set for Excitation Energy of Charge Transfer States: Systematic Investigation of Coupled Cluster Type Methods

“…Although the position of the “crossing” point is an important quantity to reproduce by approximate methods, the single point calculations presented in this work do not allow us to draw a conclusion about the ability of the methods to describe the geometry dependence of the coupling between CT and local states. In a subsequent study 72 we will investigate this problem in detail by comparing the potential energy surfaces obtained by different methods.…”

Improved Description of Charge Transfer Potential Energy Surfaces via Spin-Component Scaled CC2 and ADC(2) Methods

“…Our investigations, based on a set of local and CT states of bimolecular complexes, 31 revealed a significant underestimation of the CT excitation energy at the equilibrium structure of these systems. In ref 32 it was also shown that many issues related to the inappropriate description of CT states are connected to the asymptotic behaviour of the methods, i.e. the description of the charge transfer at infinite separation of the source and destination fragments.…”