More Like this

1. Length vs velocity gauge formulation of the frequency-dependent polarizability for 1D periodic systems at coupled cluster with single and double excitations level

   https://doi.org/10.1063/5.0306772  

   Parsons, Taylor; Caricato, Marco (January 2026, The Journal of Chemical Physics)

   This study presents the implementation of the linear response function for 1D periodic systems at the coupled cluster with single and double excitations with periodic boundary conditions level (LR-CCSD-PBC) for the calculation of the frequency-dependent electric dipole–electric dipole polarizability tensor, α(ω), using the modified velocity gauge (MVG) formalism. We compare this approach with the length gauge (LG) formalism that we presented in a previous study [Caricato et al., APL Comput. Phys. 1, 026107 (2025)] in terms of theoretical analysis and computational results. The calculations on molecular systems with large Dunning basis sets, including diffuse functions, show a smooth convergence toward the complete basis set limit (CBS) with both formalisms, but LG and MVG converge to different values when CCSD provides an incomplete treatment of electron correlation. With the small basis sets used in the PBC calculations, the difference between the gauge formalisms increases due to basis set incompleteness. For the PBC calculations on 1D periodic systems, the MVG formalism shows a faster convergence toward the thermodynamic limit with k sampling compared to LG. Furthermore, the MVG formalism provides perfect agreement between PBC and long molecular cluster models. This is not the case for LG due to the so-called missing integer issue. However, a comparison with the molecular data suggests that the LG-PBC results with the small basis set affordable in this study are closer to the CBS results than those with MVG. This study represents a further step forward in the simulation of optical response properties for solid-state materials with systematically improvable quantum mechanical methods. 

   more » « less
2. First Principles Simulations of Optical Rotation of Chiral Molecular Crystals

   https://doi.org/10.1002/chir.23709  

   Forson, Emmanuel; Parsons, Taylor; Caricato, Marco (August 2024, Chirality)

   ABSTRACT In this work, we present simulations of the optical rotation (OR) for five molecular crystals at density functional theory level with periodic boundary conditions (DFT‐PBC). Calculations are compared with experimental measurements and show semi‐quantitative agreement with experimental data for three of the crystals: tartatic acid, benzil, and pentaerythritol. For the other two crystals, aspartic acid and glutamic acid, the calculated data are in qualitative agreement with, but two orders of magnitude smaller than, the experimental data. We provide some arguments that support the theoretical predictions and suggest that the experiments should be revisited. We also find that the position of H centers provided in experimental X‐ray data is not sufficiently reliable for simulating OR, and better results are obtained when H atoms are allowed to relax while keeping heavier elements fixed at the experimental positions. Comparison with molecular cluster calculations with a better functional and a larger basis set indicate that the role of intermolecular interactions (reproduced with the PBC technique) is as or more important than the choice of model chemistry. Despite the current limitations in the level of theory that can be employed, these simulations provide a promising avenue to investigate the effect of intermolecular interactions on this sensitive electronic property of molecules and materials. 

   more » « less
3. Potential energy surfaces and dynamic properties via ab initio composite and density functional approaches

   https://doi.org/10.1002/jcc.27333  

   Patel, Prajay; Chung, Joseph; Bowman, Max Aksel; Ulusoy, Inga; Wilson, Angela K (June 2024, Journal of Computational Chemistry)

   Abstract Vibrational spectroscopy enables critical insight into the structural and dynamic properties of molecules. Presently, the majority of theoretical approaches to spectroscopy employ wavefunction‐basedab initioor density functional methods that rely on the harmonic approximation. This approximation breaks down for large molecules with strongly anharmonic bonds or for molecules with large internuclear separations. An alternative to these methods involves generating molecular anharmonic potential energy surfaces (potentials) and using them to extrapolate the vibrational frequencies. This study examines the efficacy of density functional theory (DFT) and the correlation consistent Composite Approach (ccCA) in generating anharmonic frequencies from potentials of small main group molecules. Vibrational self‐consistent field Theory (VSCF) and post‐VSCF methods were used to calculate the fundamental frequencies of these molecules from their potentials. Functional choice, basis set selection, and mode‐coupling are also examined as factors in influencing accuracy. The absolute deviations for the calculated frequencies using potentials at the ccCA level of theory were lower than the potentials at the DFT level. With DFT resulting in bending modes that are better described than those of ccCA, a multilevel DFT:ccCA approach where DFT potentials are used for single vibrational mode potentials and ccCA is used for vibrational mode‐mode couplings can be utilized for larger polyatomic systems. The frequencies obtained with this multilevel approach using VCIPSI‐PT2 were closer to experimental frequencies than the scaled harmonic frequencies, indicating the success of utilizing post‐VSCF methods to generate more accurate representations of computed infrared spectra. 

   more » « less
4. Molecular g -tensors from spin–orbit quasidegenerate N -electron valence perturbation theory: Benchmarks, intruder-state mitigation, and practical guidelines

   https://doi.org/10.1063/5.0330438  

   Chiang, Nicholas Yiching; Majumder, Rajat; Sokolov, Alexander Yu (May 2026, The Journal of Chemical Physics)

   Accurate prediction of molecular g-tensors for open-shell systems requires a balanced treatment of multireference electron correlation and relativistic spin–orbit coupling. Here, we develop and benchmark spin–orbit quasidegenerate second-order N-electron valence perturbation theory (SO-QDNEVPT2) for g-tensor calculations, treating dynamical correlation and spin–orbit effects consistently within a multistate effective Hamiltonian (EH) framework. Two g-tensor approaches are implemented: a spin-free EH approach based on second-order response and a Kramers (K) approach that extracts g from spin-mixed SO-QDNEVPT2 states. We assess their performance on a benchmark set of 23 molecules spanning diatomics and small polyatomics, low- to high-spin species, and weak to strong spin–orbit coupling. Across the dataset, SO-QDNEVPT2 improves agreement with experiment relative to state-averaged complete active-space self-consistent field. The EH and K formalisms agree for modest g-shifts, but the K approach becomes essential when the shifts become large. We demonstrate that QDNEVPT2 results can be sensitive to intruder-state instabilities that can be effectively mitigated with level-shift or renormalization techniques. We then analyze the dependence of SO-QDNEVPT2 results on key computational parameters, including active space, number of states, state-averaging weights, gauge origin, and basis set. These results establish SO-QDNEVPT2 as a robust framework for computing g-tensors in correlated, relativistic open-shell molecules, offering practical guidelines for its applications. 

   more » « less
5. Spin–orbit couplings within spin-conserving and spin-flipping time-dependent density functional theory: Implementation and benchmark calculations

   https://doi.org/10.1063/5.0130868  

   Kotaru, Saikiran; Pokhilko, Pavel; Krylov, Anna I. (December 2022, The Journal of Chemical Physics)

   We present a new implementation for computing spin–orbit couplings (SOCs) within a time-dependent density-functional theory (TD-DFT) framework in the standard spin-conserving formulation as well in the spin–flip variant (SF-TD-DFT). This approach employs the Breit–Pauli Hamiltonian and Wigner–Eckart’s theorem applied to the reduced one-particle transition density matrices, together with the spin–orbit mean-field treatment of the two-electron contributions. We use a state-interaction procedure and compute the SOC matrix elements using zero-order non-relativistic states. Benchmark calculations using several closed-shell organic molecules, diradicals, and a single-molecule magnet illustrate the efficiency of the SOC protocol. The results for organic molecules (described by standard TD-DFT) show that SOCs are insensitive to the choice of the functional or basis sets, as long as the states of the same characters are compared. In contrast, the SF-TD-DFT results for small diradicals (CH 2 , [Formula: see text], SiH 2 , and [Formula: see text]) show strong functional dependence. The spin-reversal energy barrier in a Fe(III) single-molecule magnet computed using non-collinear SF-TD-DFT (PBE0, ωPBEh/cc-pVDZ) agrees well with the experimental estimate. 

   more » « less