a) Development of resonance-Raman capabilities in Q-Chem

The debate on the structure of the aqueous electron has been revived in recent years. Spectroscopic features attributed to this species, such as the resonance Raman spectrum of e−(aq), have been successfully reproduced by a model that does not carve out an excluded volume in the structure of liquid water. This has fueled the ongoing debate over the settled question of the electron's structure. We conducted quantum chemistry calculations and found that both cavity and non-cavity models reproduce certain features of the solvated electron, but only the cavity model reproduces the experimentally observed splitting of the H-O-D bend. Our findings support the cavity model as being more consistent with the measured resonance Raman spectrum.

b) Development of Femtosecond stimulated Raman spectroscopy technique in Q-Chem

Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast technique that can effectively demonstrate the spectral dynamics of an excited state. However, it has not been simulated with theoretical calculations from the first principles of quantum chemistry until now. In this study, we integrated ab-initio Molecular Dynamics (AIMD) snapshots with excited state resonance Raman calculations in the time-integrated normal mode framework to substantiate experimental FSRS spectra. Our formalism allows us to understand the time-dependent evolution of specific vibrational modes. Using oligothiophene and its derivatives as probes, we studied FSRS and observed how the mode frequencies are coupled to the torsional dihedral, reflecting excited-state relaxation toward a planar conformation. This work serves as a theoretical analogy of experimental FSRS spectroscopy, reproducing the observations and aiding in the assignment of vibrational modes.

Quantum-chemical studies of enzymatic reaction mechanisms sometimes use truncated active-site models as simplified alternatives to mixed quantum mechanics/molecular mechanics (QM/MM) procedures. Eliminating the MM degrees of freedom reduces the complexity of the sampling problem, but the trade-off is the need to introduce geometric constraints in order to prevent structural collapse of the model system during geometry optimizations that do not contain a full protein backbone. These constraints may impair the efficiency of the optimization, and care must be taken to avoid artifacts such as imaginary vibrational frequencies. We introduced a simple alternative in which terminal atoms of the model system are placed in soft harmonic confining potentials rather than being rigidly constrained. This modification is simple to implement and straightforward to use in vibrational frequency calculations, unlike iterative constraint-satisfaction algorithms, and allows the optimization to proceed without constraint even though the practical result is to fix the anchor atoms in space. The new approach is more efficient for optimizing minima and transition states, as compared to the use of fixed-atom constraints, and also more robust against unwanted imaginary-frequencies. We illustrated the method by application to several enzymatic reaction pathways where entropy makes a significant contribution to the relevant reaction barriers. The use of confining-potentials correctly describes reaction paths, and facilitates calculation of both vibrational zero-point and finite-temperature entropic corrections to barrier heights. 

Density-functional approximations developed in the past decade necessitate the use of quadrature grids that are far more dense than those required to integrate older generations of functionals. This category of difficult-to-integrate functionals includes meta-generalized gradient approximations, which depend on orbital gradients and/or the Laplacian of the density, as well as functionals based on B97 and the popular “Minnesota" class of functionals, each of which contain complicated and/or oscillatory expressions for the exchange inhomogeneity factor. We introduced two higher-quality grids that we designated SG-2 and SG-3, obtaineded by systematically “pruning" medium (75,302) and high-quality (99,590) atom-centered grids. The pruning procedure affords computational speedups approaching a factor of two for hybrid functionals applied to systems ~100 atoms, without significant loss of accuracy. The grid dependence of several popular density functionals was characterized for various properties like atomisation-energies, vibrational frequencies, non-covalent interaction energies, non-linear optical properties.

Density functional theory (DFT) is widely used for electronic structure calculations due to its simplicity and cost-effectiveness. However, the accuracy of DFT depends on the choice of density functional approximation (DFA) and the electron density it produces. SCAN is a modern functional that satisfies constraints for meta-GGA functionals but can have density-driven errors in systems like water. Density-corrected DFT (DC-DFT) improves accuracy by using the electron density from Hartree-Fock theory and elevates it towards the CCSD(T) limit. We conducted extensive calculations to evaluate the accuracy of DC-SCAN in aqueous systems. DC-SCAN consistently reproduced reference data with minimal loss of accuracy. Density-driven errors in ionic aqueous clusters were investigated, revealing spurious electron transfers. Comparisons with the FLOSIC method showed that DC-SCAN is more accurate in reducing density-driven errors. HF density is better for noncompact water clusters, while SCAN is superior for compact water molecules with 10 electrons.

Through our research, we have discovered that the accuracy of data-driven MB-DFT simulations in the liquid phase strongly depends on the chosen density-functional. Our investigations have revealed that by purifying the electron-density for each many-body contribution, we can significantly enhance the overall accuracy. Furthermore, our findings highlight the remarkable accuracy improvement achieved by applying density-correction to the SCAN functional. This advancement allows the SCAN functional to achieve accuracy comparable to the renowned coupled-cluster theory, which is known for its high level of chemical accuracy. Expanding on these advancements, we developed MB-SCAN(DC), a data-driven many-body model capable of accurately reproducing coupled-cluster reference energies for gas-phase water clusters. Crucially, our molecular dynamics simulations employing the cost-effective MB-SCAN(DC) model, which scales linearly with system size, have demonstrated excellent agreement with experimental data regarding the properties of liquid water. This strong agreement provides compelling evidence that MB-SCAN(DC) is the first DFT-based model that accurately describes water behavior from the gas to the liquid phase.