Interstate vibronic coupling constants between electronic excited states for complex molecules

In the construction of diabatic vibronic Hamiltonians for quantum dynamics in the excited-state manifold of molecules, the coupling constants are often extracted solely from information on the excited-state energies. Here, a new protocol is applied to get access to the interstate vibronic coupling constants at the time-dependent density functional theory level through the overlap integrals between excited-state adiabatic auxiliary wavefunctions. We discuss the advantages of such method and its potential for future applications to address complex systems, in particular, those where multiple electronic states are energetically closely lying and interact. We apply the protocol to the study of prototype rhenium carbonyl complexes [Re(CO)3(N,N)(L)]n+ for which non-adiabatic quantum dynamics within the linear vibronic coupling model and including spin-orbit coupling have been reported recently

Highly efficient surface hopping dynamics using a linear vibronic coupling model.

We report an implementation of the linear vibronic coupling (LVC) model within the surface hopping dynamics approach and present utilities for parameterizing this model in a blackbox fashion. This results in an extremely efficient method to obtain qualitative and even semi-quantitative information about the photodynamical behavior of a molecule, and provides a new route toward benchmarking the results of surface hopping computations. The merits and applicability of the method are demonstrated in a number of applications. First, the method is applied to the SO2 molecule showing that it is possible to compute its absorption spectrum beyond the Condon approximation, and that all the main features and timescales of previous on-the-fly dynamics simulations of intersystem crossing are reproduced while reducing the computational effort by three orders of magnitude. The dynamics results are benchmarked against exact wavepacket propagations on the same LVC potentials and against a variation of the electronic structure level. Four additional test cases are presented to exemplify the broader applicability of the model. The photodynamics of the isomeric adenine and 2-aminopurine molecules are studied and it is shown that the LVC model correctly predicts ultrafast decay in the former and an extended excited-state lifetime in the latter. Futhermore, the method correctly predicts ultrafast intersystem crossing in the modified nucleobase 2-thiocytosine and its absence in 5-azacytosine while it fails to describe the ultrafast internal conversion to the ground state in the latter

Challenges in simulating light-induced processes in DNA

© 2016 by the authors; licensee MDPI, Basel, Switzerland. In this contribution, we give a perspective on the main challenges in performing theoretical simulations of photoinduced phenomena within DNA and its molecular building blocks. We distinguish the different tasks that should be involved in the simulation of a complete DNA strand subject to UV irradiation: (i) stationary quantum chemical computations; (ii) the explicit description of the initial excitation of DNA with light; (iii) modeling the nonadiabatic excited state dynamics; (iv) simulation of the detected experimental observable; and (v) the subsequent analysis of the respective results. We succinctly describe the methods that are currently employed in each of these steps. While for each of them, there are different approaches with different degrees of accuracy, no feasible method exists to tackle all problems at once. Depending on the technique or combination of several ones, it can be problematic to describe the stacking of nucleobases, bond breaking and formation, quantum interferences and tunneling or even simply to characterize the involved wavefunctions. It is therefore argued that more method development and/or the combination of different techniques are urgently required. It is essential also to exercise these new developments in further studies on DNA and subsystems thereof, ideally comprising simulations of all of the different components that occur in the corresponding experiments

Surface hopping dynamics including intersystem crossing using the algebraic diagrammatic construction method

© 2017 Author(s). We report an implementation for employing the algebraic diagrammatic construction to second order [ADC(2)] ab initio electronic structure level of theory in nonadiabatic dynamics simulations in the framework of the SHARC (surface hopping including arbitrary couplings) dynamics method. The implementation is intended to enable computationally efficient, reliable, and easy-to-use nonadiabatic dynamics simulations of intersystem crossing in organic molecules. The methodology is evaluated for the 2-thiouracil molecule. It is shown that ADC(2) yields reliable excited-state energies, wave functions, and spin-orbit coupling terms for this molecule. Dynamics simulations are compared to previously reported results using high-level multi-state complete active space perturbation theory, showing favorable agreement

A Static Picture of the Relaxation and Intersystem Crossing Mechanisms of Photoexcited 2‑Thiouracil

Accurate excited-state quantum chemical calculations on 2-thiouracil, employing large active spaces and up to quadruple-ζ quality basis sets in multistate complete active space perturbation theory calculations, are reported. The results suggest that the main relaxation path for 2-thiouracil after photoexcitation should be S₂ → S₁ → T₂ → T₁, and that this relaxation occurs on a subpicosecond time scale. There are two deactivation pathways from the initially excited bright S₂ state to S₁, one of which is nearly barrierless and should promote ultrafast internal conversion. After relaxation to the S₁ minimum, small singlet–triplet energy gaps and spin–orbit couplings of about 130 cm^–1 are expected to facilitate intersystem crossing to T₂, from where very fast internal conversion to T₁ occurs. An important finding is that 2-thiouracil shows strong pyramidalization at the carbon atom of the thiocarbonyl group in several excited states

QBtopIC: Classification and Analysis of Excited-state Wavefunctions in Transition Metal Complexes

Slides for QBtopIChttps://www.qbicsoc.org/qbtopics.htmlEvent on 29 September 2021.</div

Intersystem Crossing Pathways in the Noncanonical Nucleobase 2‑Thiouracil: A Time-Dependent Picture

The deactivation mechanism after ultraviolet irradiation of 2-thiouracil has been investigated using nonadiabatic dynamics simulations at the MS-CASPT2 level of theory. It is found that after excitation the S₂ quickly relaxes to S₁, and from there intersystem crossing takes place to both T₂ and T₁ with a time constant of 400 fs and a triplet yield above 80%, in very good agreement with recent femtosecond experiments in solution. Both indirect S₁ → T₂ → T₁ and direct S₁ → T₁ pathways contribute to intersystem crossing, with the former being predominant. The results contribute to the understanding of how some noncanonical nucleobases respond to harmful ultraviolet light, which could be relevant for prospective photochemotherapeutic applications

The influence of the electronic structure method on intersystem crossing dynamics. The case of thioformaldehyde

The ability of different electronic structure methods to describe correctly intersystem crossing dynamics is evaluated, using thioformaldehyde as a test case. Mischievously, all methods considered—ranging from the multi-reference methods MRCISD, MS-CASPT2, or SA-CASSCF, to the single-reference methods ADC(2), CC2, and TDDFT in different flavours— provide the same state ordering and energies of the low-lying singlet and triplet electronic excited states within an acceptable error of 0.2–0.3 eV. However, the outcome of the non-adiabatic simulations after excitation to the lowest S1 (1nπ∗) state are dramatically different. While MS-CASPT2, ADC(2), BP86, and PBE do not transfer population to the triplet states within 500 fs—in consonance with experimental evidence—SA-CASSCF, B3LYP, and BHHLYP predict intersystem crossing yields between 3% and 21% within the same time. The different excited state dynamics can be rationalized by inspecting potential energy profiles along the C–S bond stretch mode and single-triplet energy gaps. It is found that already at a C–S bond length of 1.9Å, all the single-reference methods struggle to describe the correct asymptotic behavior of the potentials. Moreover, some methods, including SACASSCF, obtain incorrect 1nπ∗ − 3ππ∗ energy gaps, leading to compensation of errors (ADC(2), BP86, PBE), or wrong dynamics (SA-CASSCF, B3LYP, BHHLYP). Only the accurate MRCISD and MS-CASPT2 methods are able to describe the C–S bond correctly and thus able to deliver the correct potential energy surfaces and dynamics for the right reason. A correlation with the amount of Hartree-Fock exchange in the density functional and the easiness to access the 3ππ∗ state from the 1nπ∗ is able to explain the different behavior observed for GGA and hybrid functionals. It is thus illustrated that even in the case of a simple molecule, like CH2S, the sole assessment of vertical excitation energies as reliability predictors for non-adiabatic is inadequate. The reason is that ISC does not occur at the FC geometry, but rather at distorted geometries where the singlet-triplet gaps become small. Hence, a characterization of the potential energy surfaces beyond the Franck-Condon region is mandatory

Surface hopping dynamics on vibronic coupling models

CONSPECTUS: The simulation of photoinduced non-adiabatic dynamics is of great relevance in many scientific disciplines, ranging from physics and materials science to chemistry and biology. Upon light irradiation, different relaxation processes take place in which electronic and nuclear motion are intimately coupled. These are best described by the time-dependent molecular Schrödinger equation, but its solution poses fundamental practical challenges to contemporary theoretical chemistry. Two widely used and complementary approaches to this problem are multiconfigurational time-dependent Hartree (MCTDH) and trajectory surface hopping (SH). MCTDH is an accurate fully quantum-mechanical technique but often is feasible only in reduced dimensionality, in combination with approximate vibronic coupling (VC) Hamiltonians, or both (i.e., reduced-dimensional VC potentials). In contrast, SH is a quantum−classical technique that neglects most nuclear quantum effects but allows nuclear dynamics in full dimensionality by calculating potential energy surfaces on the fly. If nuclear quantum effects do not play a central role and a linear VC (LVC) Hamiltonian is appropriatee.g., for stiff molecules that generally keep their conformation in the excited statethen it seems advantageous to combine the efficient LVC and SH techniques. In this Account, we describe how surface hopping based on an LVC Hamiltonian (SH/LVC)as recently implemented in the SHARC surface hopping packagecan provide an economical and automated approach to simulate nonadiabatic dynamics. First, we illustrate the potential of SH/LVC in a number of showcases, including intersystem crossing in SO2, intra-Rydberg dynamics in acetone, and several photophysical studies on large transition-metal complexes, which would be much more demanding or impossible to perform with other methods. While all of the applications provide very useful insights into lightinduced phenomena, they also hint at difficulties faced by the SH/LVC methodology that need to be addressed in the future. Second, we contend that the SH/LVC approach can be useful to benchmark SH itself. By the use of the same (LVC) potentials as MCTDH calculations have employed for decades and by relying on the efficiency of SH/LVC, it is possible to directly compare multiple SH test calculations with a MCTDH reference and ponder the accuracy of various correction algorithms behind the SH methodology, such as decoherence corrections or momentum rescaling schemes. Third, we demonstrate how the efficiency of SH/ LVC can also be exploited to identify essential nuclear and electronic degrees of freedom to be employed in more accurate MCTDH calculations. Lastly, we show that SH/LVC is able to advance the development of SH protocols that can describe nuclear dynamics including explicit laser fieldsa very challenging endeavor for trajectory-based schemes. To end, this Account compiles the typical costs of contemporary SH simulations, evidencing the great advantages of using parametrized potentials. The LVC model is a sleeping beauty that, kissed by SH, is fueling the field of excited-state molecular dynamics. We hope that this Account will stimulate future research in this direction, leveraging the advantages of the SH/VC schemes to larger extents and extending their applicability to uncharted territories

Strong influence of decoherence corrections and momentum rescaling in surface hopping dynamics of transition metal complexes

The reliability of different parameters in the surface hopping method is assessed for a vibronic coupling model of a challenging transition metal complex, where a large number of electronic states of different multiplicities are met within a small energy range. In particular, the effect of two decoherence correction schemes and of various strategies for momentum rescaling and treating frustrating hops during the dynamics is investigated and compared against an accurate quantum dynamics simulation. The results show that surface hopping is generally able to reproduce the reference but also that small differences in the protocol used can strongly affect the results. We find a clear preference for momentum rescaling along only one degree of freedom, using either the nonadiabatic coupling or the gradient difference vector, and trace this effect back to an enhanced number of frustrated hops. Furthermore, reflection of the momentum after frustrated hops is shown to work better than to ignore the process completely. The study also highlights the importance of the decoherence correction but neither of the two methods employed, energy based decoherence or augmented fewest switches surface hopping, performs completely satisfactory and we trace this effect back to a lack of size-consistency. Finally, the effect of different methods for analysing the populations is highlighted. More generally, the study emphasises the importance of the often neglected parameters in surface hopping and shows that there is still need for simple, robust, and generally applicable correction schemes