Protonation state control of electric field induced molecular switching mechanisms.

Scanning tunneling microscopy tip-induced deprotonation has been demonstrated experimentally and can be used as an additional control mechanism in electric-field induced molecular switching. The goal of the current work is to establish whether (de)protonation can be used to inhibit or enhance the electric field controlled thermal and photoisomerization processes. Dihydroxyazobenzene is used as a model system, where protonation/deprotonation of the free hydroxyl moiety changes the azo bond order, and so modifies the rate of electric field induced isomerization. Through the combined action of deprotonation and applied field, it was found that the cis-to-trans thermal isomerization barrier could be completely removed, changing the isomerization half-life from the order of several months. In addition, due to the presence of multiple isomerization mechanisms, electric fields could modify the isomerization kinetics by increasing the number of energetically viable isomerization pathways, rather than reducing the activation barrier of the lowest energy pathway. Excited state calculations indicated that the protonation state and electric field could be used together to control the presence of electronic degeneracies along the rotation pathway between S0/S1, and along all three pathways between S1/S2. This work provides insight into the mechanisms that enable the use of protonation state, light, and electric fields in concert to control molecular switches.

Role of Exact Exchange in Difference Projected Double-Hybrid Density Functional Theory for Treatment of Local, Charge Transfer, and Rydberg Excitations.

Difference approaches to the study of excited states have undergone a renaissance in recent years, with the development of a plethora of algorithms for locating self-consistent field approximations to excited states. Density functional theory is likely to offer the best balance of cost and accuracy for difference approaches, and yet there has been little investigation of how the parametrization of density functional approximations affects performance. In this work, we aim to explore the role of the global Hartree-Fock exchange parameter in tuning accuracy of different excitation types within the framework of the recently introduced difference projected double-hybrid density functional theory approach and contrast the performance with conventional time-dependent double-hybrid density functional theory. Difference projected double-hybrid density functional theory was demonstrated to give vertical excitation energies with average error and standard deviation with respect to multireference perturbation theory comparable to more expensive linear-response coupled cluster approaches ( J. Chem. Phys.2020, 153, 074103). However, despite benchmarking of local excitations, there has been no investigation of the methods performance for charge transfer or Rydberg excitations. In this work we report a new benchmark of charge transfer, Rydberg, and local excited state vertical excitation energies and examine how the exact Hartree-Fock exchange affects the benchmark performance to provide a deeper understanding of how projection and nonlocal correlation balance differing sources of error in the ground and excited states.

Nonorthogonal Active Space Decomposition of Wave Functions with Multiple Correlation Mechanisms.

The nonorthogonal active space decomposition (NO-ASD) methodology is proposed for describing systems containing multiple correlation mechanisms. NO-ASD partitions the wave function by a correlation mechanism, such that the interactions between different correlation mechanisms are treated with an effective Hamiltonian approach, while interactions between correlated orbitals in the same correlation mechanism are treated explicitly. As a result, the determinant expansion scales polynomially with the number of correlation mechanisms rather than exponentially, which significantly reduces the factorial scaling associated with the size of the correlated orbital space. Despite the nonorthogonal framework of NO-ASD, the approach can take advantage of computational efficient matrix element evaluation when performing nonorthogonal coupling of orthogonal determinant expansions. In this work, we introduce and examine the NO-ASD approach in comparison to complete active space methods to establish how the NO-ASD approach reduces the problem dimensionality and the extent to which it affects the amount of correlation energy recovered. Calculations are performed on ozone, nickel-acetylene, and isomers of µ-oxo dicopper ammonia.