Jonathan Fajen

All Publications

• Perspective on a challenge: Predicting the photochemistry of cyclobutanone. The Journal of chemical physics Janoš, J., List, N. H., Orr-Ewing, A. J., Suchan, J., Barbatti, M., Bennett, O., Brady, M., Carmona-García, J., Crespo-Otero, R., Eng, J., Fajen, O. J., Garavelli, M., Gómez, S., Green, A. E., Hernández, F. J., Hollas, D., Hutton, L., Ibele, L. M., Kirrander, A., Lan, Z., Lassmann, Y., Lawrence, J. E., Levine, B. G., Makhov, D. V., Mannouch, J. R., Miao, X., Mitrić, R., Parker, S. M., Penfold, T. J., Peng, J., Richardson, J. O., Shalashilin, D., Slavíček, P., Spinlove, K. E., Vindel-Zandbergen, P., Agostini, F., Bonella, S., Martínez, T. J., Worth, G. A., Curchod, B. F. 2026; 165 (2)

  Abstract

  This Perspective is part of a Special Topic that explored the maturity of nonadiabatic molecular dynamics for predicting photochemical processes. In 2023, a prediction challenge was issued to the community of computational photochemists to simulate the photochemistry of cyclobutanone, photoexcited at 200 nm, and the resulting time-resolved mega-electronvolt ultrafast electron diffraction (MeV-UED) signal. The challenge attracted 15 theoretical predictions from more than 70 researchers, employing a wide range of strategies for electronic structure and nonadiabatic molecular dynamics to predict the time-resolved MeV-UED signal before the experiment had been conducted at SLAC (Stanford, USA). The MeV-UED instrument at Shanghai Jiao Tong University was also used to provide a second independent time-resolved MeV-UED signal for the photochemistry of cyclobutanone. This Perspective discusses the various approaches and strategies used by the participants to predict the photochemistry of cyclobutanone. This work also summarizes the strengths and weaknesses of various methods used for photoexcitation, electronic structure, nonadiabatic dynamics, and calculation of observables, as agreed by the participants during a CECAM workshop dedicated to the results of the challenge and organized in Lausanne in April 2025. This Perspective also collects all the predicted time-resolved MeV-UED signals into a single figure, together with the experimental signal. The challenge (i) demonstrated the qualitative predictive power of nonadiabatic molecular dynamics and (ii) underscored the impact of electronic-structure theory on the outcome of the excited-state dynamics and the need for its careful benchmarking. This effort allowed the community to share practical strategies to perform nonadiabatic dynamics (discussed in the present Perspective) and constitutes a "calibration" exercise for computational photochemistry.

  View details for DOI 10.1063/5.0338792

  View details for PubMedID 42446072

• Shadow excited state molecular dynamics with the ΔSCF method. The Journal of chemical physics Fajen, O. J., Grånäs, O., Martínez, T. J., Niklasson, A. M. 2026; 164 (9)

  Abstract

  We present an extension of the shadow extended Lagrangian Born-Oppenheimer molecular dynamics method to excited state molecular dynamics (ESMD) in the context of ΔSCF Kohn-Sham density functional theory, with demonstrations performed using self-consistent charge density functional tight binding (SCC-DFTB) theory. In this shadow ESMD approach, the approximate iterative solution to the exact potential in conventional ESMD is replaced by an exact single-step solution to an approximate shadow excited-state potential. The energy functional that defines this shadow excited-state potential as a stationary (non-aufbau) solution is obtained from a linearization about an approximate excited state density, which would become a progressively worse approximation as the dynamics ensue if it were static. To avoid this, we propagate the approximate excited-state (charge) density as an additional dynamical variable in an extended Lagrangian approach. We show that, in addition to offering significant improvement in computational cost relative to direct ESMD, our shadow ESMD method provides enhanced stability and robustness relative to its "exact" counterpart. Our implementation is carried out in the context of SCC-DFTB theory but should be broadly generalizable, both to ab initio electronic structure methods and to other semi-empirical quantum chemistry approaches.

  View details for DOI 10.1063/5.0293146

  View details for PubMedID 41784448

• Accelerating CCSD(T) on Graphical Processing Units (GPUs). The journal of physical chemistry. A Fajen, O. J., Kelly, J. E., Hohenstein, E. G., Martinez, T. J. 2026

  Abstract

  Coupled cluster with singles, doubles, and perturbative triples (CCSD(T)) often provides ground state correlation energies within ″chemical accuracy", but it suffers from high computational cost and steep scaling with system size. We present a GPU-accelerated implementation of CCSD(T) in the TeraChem software package. The new implementation achieves state-of-the-art performance, enabling the calculation of the (T) correction for a system with 63 atoms and more than 1000 basis functions in a little under 8 h on a single node. Additionally, we demonstrate the utility of our optimized implementation for the rapid calculation of full CCSD(T)/CBS stacking energies for all ten unique DNA base pair stacked tetramers. We expect that the TeraChem CCSD(T) implementation will enable the rapid calculation of high-level data that was not previously accessible in a reasonable time frame.

  View details for DOI 10.1021/acs.jpca.5c08113

  View details for PubMedID 41747765

• Photoinduced hydrogen dissociation in thymine predicted by coupled cluster theory. Nature communications Kjønstad, E. F., Fajen, O. J., Paul, A. C., Angelico, S., Mayer, D., Gühr, M., Wolf, T. J., Martínez, T. J., Koch, H. 2024; 15 (1): 10128

  Abstract

  The fate of thymine upon excitation by ultraviolet radiation has been the subject of intense debate. Today, it is widely believed that its ultrafast excited state gas phase decay stems from a radiationless transition from the bright ππ* state to a dark nπ* state. However, conflicting theoretical predictions have made the experimental data difficult to interpret. Here we simulate the early gas phase ultrafast dynamics in thymine at the highest level of theory to date. This is made possible by performing wavepacket dynamics with a recently developed coupled cluster method. Our simulation confirms an ultrafast ππ* to nπ* transition (τ = 41 ± 14 fs). Furthermore, the predicted oxygen-edge X-ray absorption spectra agree quantitatively with experiment. We also predict an as-yet uncharacterized πσ* channel that leads to hydrogen dissociation at one of the two N-H bonds. Similar behavior has been identified in other heteroaromatic compounds, including adenine, and several authors have speculated that a similar pathway may exist in thymine. However, this was never confirmed theoretically or experimentally. This prediction calls for renewed efforts to experimentally identify or exclude the presence of this channel.

  View details for DOI 10.1038/s41467-024-54436-2

  View details for PubMedID 39578441

  View details for PubMedCentralID PMC11584849

• Extending GPU-accelerated Gaussian integrals in the TeraChem software package to f type orbitals: Implementation and applications. The Journal of chemical physics Wang, Y., Hait, D., Johnson, K. G., Fajen, O. J., Zhang, J. H., Guerrero, R. D., Martinez, T. J. 2024; 161 (17)

  Abstract

  The increasing availability of graphics processing units (GPUs) for scientific computing has prompted interest in accelerating quantum chemical calculations through their use. However, the complexity of integral kernels for high angular momentum basis functions often limits the utility of GPU implementations with large basis sets or for metal containing systems. In this work, we report the implementation of f function support in the GPU-accelerated TeraChem software package through the development of efficient kernels for the evaluation of Hamiltonian integrals. The high efficiency of the resulting code is demonstrated through density functional theory (DFT) calculations on increasingly large organic molecules and transition metal complexes, as well as coupled cluster singles and doubles calculations on water clusters. Preliminary investigations into Ni(I) catalysis with DFT and the photochemistry of MnH(CH3) with complete active space self-consistent field are also carried out. Overall, our GPU-accelerated software appears to be well-suited for fast simulation of large transition metal containing systems, as well as organic molecules.

  View details for DOI 10.1063/5.0233523

  View details for PubMedID 39503473

• Prediction of photodynamics of 200 nm excited cyclobutanone with linear response electronic structure and ab initio multiple spawning. The Journal of chemical physics Hait, D., Lahana, D., Fajen, O. J., Paz, A. S., Unzueta, P. A., Rana, B., Lu, L., Wang, Y., Kjønstad, E. F., Koch, H., Martínez, T. J. 2024; 160 (24)

  Abstract

  Simulations of photochemical reaction dynamics have been a challenge to the theoretical chemistry community for some time. In an effort to determine the predictive character of current approaches, we predict the results of an upcoming ultrafast diffraction experiment on the photodynamics of cyclobutanone after excitation to the lowest lying Rydberg state (S2). A picosecond of nonadiabatic dynamics is described with ab initio multiple spawning. We use both time dependent density functional theory (TDDFT) and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory for the underlying electronic structure theory. We find that the lifetime of the S2 state is more than a picosecond (with both TDDFT and EOM-CCSD). The predicted ultrafast electron diffraction spectrum exhibits numerous structural features, but weak time dependence over the course of the simulations.

  View details for DOI 10.1063/5.0203800

  View details for PubMedID 38912674