arXiv subjects

Marcus D. Liebenthal

Publications and source records attributed to Marcus D. Liebenthal.

6 recordsLinked to original sources

arXiv2025

Relativistic Two-component Double Ionization Potential Equation-of-Motion Coupled Cluster with the Dirac--Coulomb--Breit Hamiltonian

We have implemented relativistic formulations of DIP-EOMCCSD and DIP-EOMCCSDT within the 1eX2C and DC-, DCG-, and DCB-X2C frameworks. Direct comparisons against full 4c-DIP-EOMCCSD calculations show excellent agreement with DC(G)-X2C-DIP-EOMCCSD, suggesting, at least for the systems studied herein, two-electron relativistic effects are well-described by the mean-field treatment in mmfX2C, and remaining relativistic two-electron and electron-positron correlation effects are negligible. A subsequent basis set study on vertical double IPs for noble gas and diatomic species has shown that DCB-X2C-DIP-EOMCCSD tends to overestimate double IP values in the limit of a complete one-electron basis, by more than 0.25 eV, on average. For atomic systems, we were able to demonstrate that a composite scheme whereby the dominant correlation effects are captured by large-basis DCB-X2C-DIP-EOMCCSD and remaining high-order correlation effects are approximately modeled via small-basis DCB-X2C-DIP-EOMCCSDT brings the double IP values into excellent agreement with experiment; for Xe atom, for example, absolute errors in double IP values from this approach are less than 0.02 eV. However, we found the ANO-RCC family of basis sets used in our composite approach to have poor convergence behavior in terms of DCB-X2C-DIP-EOMCC calculations, as the estimates computed using the non-relativistic DIP-EOMCC approach at the large basis set limit indicates a larger 0.1--0.2 eV error relative to experimental data.

physics.chem-ph↗

arXiv2025

Automated Quantum Chemistry Code Generation with the p$^\dagger$q Package

This article summarizes recent updates to the p$^\dagger$q package, which is a C++ accelerated Python library for generating equations and computer code corresponding to singly-reference many-body quantum chemistry methods such as coupled-cluster (CC) and equation-of-motion (EOM) CC theory. Since 2021, the functionality in \pq has expanded to include boson operators, coupled fermion-boson operators, unitary cluster operators, non-particle-conserving EOM operators, spin tracing, multiple single-particle subspaces, and more. Additional developments allow for the generation of C++ and Python code that minimizes floating-point operations via contraction order optimization, sub-expression elimination, and the fusion of similar terms.

physics.chem-ph↗

arXiv2024

The orientation dependence of cavity-modified chemistry

Recent theoretical studies have explored how ultra-strong light--matter coupling can be used as a handle to control chemical transformations. {\em Ab initio} cavity quantum electrodynamics (QED) calculations demonstrate that large changes to reaction energies or barrier heights can be realized by coupling electronic degrees of freedom to vacuum fluctuations associated with an optical cavity mode, provided that large enough coupling strengths can be achieved. In many cases, the cavity effects display a pronounced orientational dependence. In this Perspective, we highlight the critical role that geometry relaxation can play in such studies. As an example, we consider recent work [Nat.~Commun.~{\bf 14}, 2766 (2023)] that explored the influence of an optical cavity on Diels-Alder cycloaddition reactions and reported large changes to reaction enthalpies and barrier heights, as well as the observation that changes in orientation can inhibit the reaction or select for one reaction product or another. Those calculations used fixed molecular geometries optimized in the absence of the cavity and fixed relative orientations of the molecules and the cavity mode polarization axis. Here, we show that, when given a chance to relax in the presence of the cavity, the molecular species reorient in a way that eliminates the orientational dependence. Moreover, in this case, we find that qualitatively different conclusions regarding the impact of the cavity on the thermodynamics of the reaction can be drawn from calculations that consider relaxed versus unrelaxed molecular structures.

physics.chem-ph↗

arXiv2023

Assessing the Effects of Orbital Relaxation and the Coherent-State Transformation in Quantum Electrodynamics Density Functional and Coupled-Cluster Theories

Cavity quantum electrodynamics (QED) generalizations of time-dependent (TD) density functional theory (DFT) and equation-of-motion (EOM) coupled-cluster (CC) theory are used to model small molecules strongly coupled to optical cavity modes. We consider two types of calculations. In the first approach (termed "relaxed"), we use a coherent-state-transformed Hamiltonian within the ground- and excited-state portions of the calculations, and cavity-induced orbital relaxation effects are included at the mean-field level. This procedure guarantees that the energy is origin invariant in post-self-consistent-field calculations. In the second approach (termed "unrelaxed"), we ignore the coherent-state transformation and the associated orbital relaxation effects. In this case, ground-state unrelaxed QED-CC calculations pick up a modest origin dependence but otherwise reproduce relaxed QED-CC results within the coherent-state basis. On the other hand, a severe origin dependence manifests in ground-state unrelaxed QED mean-field energies. For excitation energies computed at experimentally realizable coupling strengths, relaxed and unrelaxed QED-EOM-CC results are similar, while significant differences emerge for unrelaxed and relaxed QED-TDDFT. First, QED-EOM-CC and relaxed QED-TDDFT both predict that electronic states that are not resonant with the cavity mode are nonetheless perturbed by the cavity. Unrelaxed QED-TDDFT, on the other hand, fails to capture this effect. Second, in the limit of large coupling strengths, relaxed QED-TDDFT tends to overestimate Rabi splittings, while unrelaxed QED-TDDFT underestimates them, given splittings from relaxed QED-EOM-CC as a reference, and relaxed QED-TDDFT generally does the better job of reproducing the QED-EOM-CC results.

physics.chem-ph↗

arXiv2022

Equation-of-motion cavity quantum electrodynamics coupled-cluster theory for electron attachment

The electron attachment variant of equation-of-motion coupled-cluster theory (EOM-EA-CC) is generalized to the case of strong light-matter coupling within the framework of cavity quantum electrodynamics (QED). The resulting EOM-EA-QED-CC formalism provides an ab initio, correlated, and non-perturbative description of cavity-induced effects in many-electron systems that complements other recently proposed cavity-QED-based extensions of coupled-cluster theory. Importantly, this work demonstrates that QED generalizations of EOM-CC theory are useful frameworks for exploring particle non-conserving sectors of Fock space, thereby establishing a path forward for the simultaneous description of both strong electron-electron and electron-photon correlation effects.

physics.chem-ph↗

arXiv2021

Challenges for variational reduced-density-matrix theory with three-particle N-representability conditions

The direct variational optimization of the two-electron reduced density matrix (2RDM) can provide a reference-independent description of the electronic structure of many-electron systems that naturally captures strong or nondynamic correlation effects. Such variational 2RDM approaches can often provide a highly accurate description of strong electron correlation, provided that the 2RDMs satisfy at least partial three-particle $N$-representability conditions ({\em e.g.}, the T2 condition). However, recent benchmark calculations on hydrogen clusters [J. Chem. Phys. {\bf 153}, 104108 (2020)] suggest that even the T2 condition leads to unacceptably inaccurate results in the case of 2- and 3-dimensional clusters. We demonstrate that these failures persist under the application of full three-particle $N$-representability conditions (3POS). A variety of correlation metrics are explored in order to identify regimes under which 3POS calculations become unreliable, and we find that the relative squared magnitudes of the cumulant three- and two-particle reduced density matrices correlates reasonably well with the energy error in these systems. However, calculations on other molecular systems reveal that this metric is not a universal indicator for the reliability of reduced-density-matrix theory with 3POS conditions.

physics.chem-ph↗