Scott M. Garner

We present Chronus Quantum (ChronusQ) 1.0, an open-source software platform for modern electronic structure theory with particular emphasis on the molecular Dirac–Coulomb–Breit Hamiltonian, relativistic many-body methods, multicomponent quantum chemistry, and time-dependent phenomena. Rather than serving as another general-purpose quantum chemistry package, ChronusQ is designed to develop and disseminate new theoretical methodologies through reusable, interoperable, and extensible software frameworks. At the heart of the software is a unified relativistic infrastructure spanning the molecular Dirac–Coulomb–Breit Hamiltonian, exact two-component methods, atomic-orbital integral engines, Cholesky decomposition, finite-magnetic-field gauge-including atomic orbitals, and distributed tensor computations. Building upon these shared components, ChronusQ provides relativistic self-consistent-field and density functional theory, multiconfigurational methods based on the small-tensor-product distributed active space framework, relativistic coupled-cluster theory, nuclear–electronic orbital methods, and real-time quantum dynamics. This article presents the scientific capabilities, software architecture, and development philosophy of ChronusQ, illustrating how interoperable software infrastructures can accelerate the development of modern electronic structure methods while providing a flexible and sustainable platform for future advances in molecular quantum science.

Polaritons offer the possibility of modifying chemical properties through the strong coupling of molecules to photons confined in optical cavities. From a first-principles perspective, understanding how these strong light–matter interactions cause chemical changes requires quantum mechanical descriptions of electrons, nuclei, and photons. Herein, we develop the ab initio cavity quantum electrodynamic nuclear–electronic orbital complete active space configuration interaction (QED-NEO-CASCI) method, in which electrons, specified nuclei, and cavity photons are all treated quantum mechanically. Using this method, we are able to describe hydrogen tunneling under vibrational strong coupling and geometry changes in a hydrogen-bonded complex coupled to a cavity. Under vibronic strong coupling, we observe features in the polaritonic absorption spectrum that correspond to Raman active transitions outside a cavity. We also develop the real-time variant, denoted time-dependent configuration interaction (QED-NEO-TDCI). This method allows us to simulate nonadiabatic hydrogen tunneling dynamics and observe the real-time quantum mechanical transfer of energy between the molecule and cavity photons. These methods provide the ab initio foundation for studying correlated electronic–nuclear–photonic chemical physics.

Simulations of vibrational spectra are important for interpreting experimental data as well as understanding molecular structure and dynamics. Herein, we present an approach for the efficient and accurate incorporation of anharmonicity into such simulations. Real-time nuclear–electronic orbital time-dependent density functional theory treats specified protons quantum mechanically on the same level as the electrons, propagating the electronic and protonic densities according to the time-dependent Schrödinger equation. This approach inherently includes the anharmonicity of the quantum protons and can be combined with Ehrenfest dynamics for the classical nuclei. Herein, this real-time nuclear–electronic orbital (NEO)–Ehrenfest approach is combined with the quasiclassical trajectory (QCT) approach for generating initial conditions that include the zero-point energy of the classical nuclei, thereby enabling sampling of the anharmonic regions of the potential energy surface. The resulting NEO-QCT approach is shown to capture the anharmonic heavy nuclear motion, as well as the anharmonicity of the quantum protons, for a series of molecular systems, including HCN, HNC, FHF−, CH2O, and HCOOH. The NEO-QCT method also captures the distinct spectral features of the formate–water complex (CHO2−⋅ H2O), including the redshifted and broadened OH stretch band due to strong anharmonicity arising from hydrogen bonding and coupling between the motions of the hydrogen nuclei and the heavy nuclei. The NEO-QCT method enables computationally practical simulations of vibrational spectra of molecules that exhibit significant anharmonicity and coupling between vibrational modes.

Polaritons have gained significant attention for the tantalizing possibility of modifying chemical properties and dynamics by coupling molecules to resonant cavity modes to create hybrid light–matter quantum states. Herein, we implement the semiclassical nuclear–electronic orbital time-dependent configuration interaction (NEO-TDCI) approach, which treats electrons and specified nuclei on the same quantum mechanical level, while treating the cavity mode classically. This ab initio dynamics approach can describe both the electronic strong coupling and the vibrational strong coupling regimes at the same level of theory without invoking the Born–Oppenheimer separation between the quantum nuclei and the electrons. This approach is used to simulate resonant and off-resonant vibronic strong coupling, where the cavity mode couples to one or many vibronic transitions associated with joint electronic–nuclear excitations within a vibronic progression. In this case, the cavity mode couples to nuclear motions even for cavity frequencies typically associated with electronic strong coupling. This approach is also used to illustrate that coupling a molecule to a cavity mode can alter hydrogen tunneling dynamics. The semiclassical NEO-TDCI approach provides the foundation for investigating how polaritons may be able to influence chemical reactions involving tunneling and nonadiabatic effects.

Time-resolved spectroscopy is an important tool for probing photochemically induced nonequilibrium dynamics and energy transfer. Herein, a method is developed for the ab initio simulation of vibronic spectra and dynamical processes. This framework utilizes the recently developed nuclear–electronic orbital time-dependent configuration interaction (NEO-TDCI) approach, which treats all electrons and specified nuclei quantum mechanically on the same footing. A strategy is presented for calculating time-resolved vibrational and electronic absorption spectra from any initial condition. Although this strategy is general for any TDCI implementation, utilizing the NEO framework allows for the explicit inclusion of quantized nuclei, as illustrated through the calculation of vibrationally hot spectra. Time-resolved spectra produced by either vibrational or electronic Rabi oscillations capture ground-state absorption, stimulated emission, and excited-state absorption between vibronic states. This methodology provides the foundation for fully ab initio simulations of multidimensional spectroscopic experiments.

Combining real-time electronic structure with the nuclear-electronic orbital (NEO) method has enabled the simulation of complex nonadiabatic chemical processes. However, accurate descriptions of hydrogen tunneling and double excitations require multiconfigurational treatments. Herein, we develop and implement the real-time NEO time-dependent configuration interaction (NEO-TDCI) approach. Comparison to NEO-full CI calculations of absorption spectra for a molecular system shows that the NEO-TDCI approach can accurately capture the tunneling splitting associated with the electronic ground state as well as vibronic progressions corresponding to double electron-proton excitations associated with excited electronic states. Both of these features are absent from spectra obtained with single reference real-time NEO methods. Our simulations of hydrogen tunneling dynamics illustrate the oscillation of the proton density from one side to the other via a delocalized, bilobal proton wave function. These results indicate that the NEO-TDCI approach is highly suitable for studying hydrogen tunneling and other inherently multiconfigurational systems.

We theoretically investigate the influence of diradical electron spin coupling on the time-resolved X-ray absorption spectra of the photochemical ring opening of furanone. We predict geometry-dependent carbon K-edge signals involving transitions from core orbitals to both singly and unoccupied molecular orbitals. The most obvious features of the ring opening come from the carbon atom directly involved in the bond breaking through its transition to both the newly formed singly occupied and the available lowest unoccupied molecular orbitals (SOMO and LUMO, respectively). In addition to this primary feature, the singlet spin coupling of four unpaired electrons that arises in the core-to-LUMO states creates additional geometry dependence in some spectral features with both oscillator strengths and relative excitation energies varying observably as a function of the ring opening. We attribute this behavior to a spin-occupancy-induced selection rule, which occurs when singlet spin coupling is enforced in the diradical state. Notably, one of these geometry-sensitive core-to-LUMO transitions excites core electrons from a backbone carbon not involved in the bond breaking, providing a novel nonlocal X-ray probe of chemical dynamics arising from electron spin coupling.