James Shee
· Norman Hackerman-Welch Assistant Professor of ChemistryVerifiedRice University · Chemistry
Active 2015–2026
About
James Shee is a professor associated with the Department of Chemistry at Rice University. His research focuses on electronic structure theory, particularly in the context of strongly correlated electronic systems. He is actively involved in exploring stochastic methods in electronic structure theory, as evidenced by his participation in related conferences and group activities. Throughout his career, he has led a research group that includes postdoctoral fellows, graduate students, and visiting scholars, engaging in collaborative and interdisciplinary projects. His lab emphasizes innovative approaches to understanding complex electronic interactions, contributing to advancements in computational chemistry and quantum chemistry.
Research topics
- Physics
- Chemistry
- Chemical physics
- Quantum mechanics
- Computational chemistry
- Physical chemistry
- Atomic physics
- Nuclear physics
- Mathematics
- Photochemistry
- Molecular physics
- Organic chemistry
- Condensed matter physics
- Optics
Selected publications
The Curious Case of Dual Emission in 9,10-Bis(phenylethynyl)anthracene
ChemRxiv · 2026-02-09
articleOpen accessThe photophysics of molecular crystals are governed by the interplay of molecular packing, electronic coupling, and lattice disorder.9,10-Bis(phenylethynyl)anthracene (BPEA) is a benchmark system for singlet fission and solid-state triplet-triplet annihilation (TTA), yet its optical spectra display long-standing anomalies, including dual absorption and emission features that defy conventional excitonic interpretations.Here, we resolve these puzzles using steady-state and time-resolved spectroscopy combined with exciton-charge-transfer (CT) vibronic modeling, molecular dynamics simulations, and first-principles electronic structure calculations.We show that the characteristic double-band absorption of crystalline BPEA arises from electronic mixing between Frenkel excitons and low-lying CT states, rather than polymorphism or conventional H-aggregate behavior.In contrast, the anomalous low-energy emission originates from structural defects associated with Xshaped BPEA dimers, whose stabilized CT character yields emissive states decoupled from the bulk exciton manifold.These trap states act as inherent dopants to suppress singlet fission while enhancing CT-triplet-pair mixing, creating efficient TTA hotspots and directly linking packing defects to increased upconversion efficiency.
ArXiv.org · 2026-05-05
articleOpen accessPhaseless auxiliary-field quantum Monte Carlo (AFQMC) has in several cases been found to perform well on strongly correlated systems. Here, we benchmark the method for three iron-sulfur clusters ([2Fe-2S], [4Fe-4S], and the FeMo cofactor) using a hierarchy of trial states derived from coupled cluster (CC) theory, including up to quadruple excitations, as well as multi-Slater trial states derived from the density matrix renormalization group. Our results reveal for these systems that, as the symmetry-broken trial is improved, the phaseless AFQMC energy can become less accurate, and in some cases even less accurate than the underlying trial projected energy, displaying an inverted energy pattern that is only corrected once the trial fidelity is sufficiently high. For [2Fe-2S], we show that this can coincide with a simultaneous improvement in the trial state and the walker ensemble. We further find that this is not solely due to the use of spin-unrestricted trial states, as the inversion persists in [2Fe-2S] when we explicitly break the symmetry of the Hamiltonian by applying a fictitious spin-Zeeman field. Instead, we find that the energy inversion is related to the choice of measurement trial, where using a high-order CC trial state for measurements may introduce errors that are suppressed when the measurement wave function is restricted to lower excitation subspaces. In particular, measuring the energy with the mean-field reference while guiding the walkers with a CC trial improves the overall accuracy across the iron-sulfur clusters, with a possible exception for [4Fe-4S]. Taken together, our findings suggest that the relatively accurate energies obtained with an HF trial state in these systems arise from favorable error cancellation, warranting significant caution about the reliability of phaseless AFQMC with such trials for strongly correlated transition-metal systems of this kind.
The Curious Case of Dual Emission in 9,10-Bis(phenylethynyl)anthracene
Journal of the American Chemical Society · 2026-04-04
articleOpen accessThe photophysics of molecular crystals are governed by the interplay of molecular packing, electronic coupling, and lattice disorder. 9,10-Bis(phenylethynyl)anthracene (BPEA) is a benchmark system for singlet fission and solid-state triplet-triplet annihilation (TTA), yet its optical spectra display long-standing anomalies, including dual absorption and emission features that defy conventional excitonic interpretations. Here, we resolve these puzzles using steady-state and time-resolved spectroscopy combined with exciton-charge-transfer (CT) vibronic modeling, molecular dynamics simulations, and first-principles electronic structure calculations. We show that the characteristic double-band absorption of crystalline BPEA arises from electronic mixing between Frenkel excitons and low-lying CT states rather than polymorphism or conventional H-aggregate behavior. In contrast, the anomalous low-energy emission originates from structural defects associated with X-shaped BPEA dimers, whose stabilized CT character yields emissive states decoupled from the bulk exciton manifold. These trap states act as inherent dopants to suppress singlet fission while enhancing CT-triplet-pair mixing, creating efficient TTA hotspots and directly linking packing defects to increased upconversion efficiency.
arXiv (Cornell University) · 2026-05-05
preprintOpen accessPhaseless auxiliary-field quantum Monte Carlo (AFQMC) has in several cases been found to perform well on strongly correlated systems. Here, we benchmark the method for three iron-sulfur clusters ([2Fe-2S], [4Fe-4S], and the FeMo cofactor) using a hierarchy of trial states derived from coupled cluster (CC) theory, including up to quadruple excitations, as well as multi-Slater trial states derived from the density matrix renormalization group. Our results reveal for these systems that, as the symmetry-broken trial is improved, the phaseless AFQMC energy can become less accurate, and in some cases even less accurate than the underlying trial projected energy, displaying an inverted energy pattern that is only corrected once the trial fidelity is sufficiently high. For [2Fe-2S], we show that this can coincide with a simultaneous improvement in the trial state and the walker ensemble. We further find that this is not solely due to the use of spin-unrestricted trial states, as the inversion persists in [2Fe-2S] when we explicitly break the symmetry of the Hamiltonian by applying a fictitious spin-Zeeman field. Instead, we find that the energy inversion is related to the choice of measurement trial, where using a high-order CC trial state for measurements may introduce errors that are suppressed when the measurement wave function is restricted to lower excitation subspaces. In particular, measuring the energy with the mean-field reference while guiding the walkers with a CC trial improves the overall accuracy across the iron-sulfur clusters, with a possible exception for [4Fe-4S]. Taken together, our findings suggest that the relatively accurate energies obtained with an HF trial state in these systems arise from favorable error cancellation, warranting significant caution about the reliability of phaseless AFQMC with such trials for strongly correlated transition-metal systems of this kind.
arXiv (Cornell University) · 2026-05-05
preprintOpen accessSenior authorPhaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) has emerged as a promising electronic structure method for correlated electronic systems. However, the quality of its predictions depends critically on the choice of trial wavefunction, and it is not obvious how to make an optimal choice especially for strongly correlated states of large systems. Mean-field wavefunctions are compelling trial wavefunction candidates as they map directly to chemical concepts and can be obtained with $O(N^4)$ cost. Yet in the strongly correlated regime one faces a symmetry dilemma and the existence of multiple nearly-degenerate solutions. In this work we investigate active space models of [2Fe-2S]$^{2+}$, mixed-valent [4Fe-4S]$^{2+}$, and [4Fe-4S]$^{4+}$ and explore the sensitivity of ph-AFQMC to the choice of unrestricted Hartree-Fock trial wavefunction. We find that chemical properties and physical symmetries, rather than the variational energy, ought to guide the choice of mean-field trial for ph-AFQMC (or reference state for coupled cluster models), and show that surprisingly accurate ground-state energies for these systems can be obtained. However, in all cases we find a rapidly vanishing overlap between the stochastic wavefunction and the UHF trial, indicating that the trials are suboptimal importance functions. By analogy to a similar situation in the stretched helium dimer cation, we show how this sampling bias pushes ph-AFQMC towards artificially negative energies, which evidently can be compensated for by the phaseless bias in certain cases.
ffsim: Faster simulation of fermionic quantum circuits
ArXiv.org · 2026-05-04
articleOpen accessWe present ffsim, an open-source software library for fast simulation of fermionic quantum circuits. ffsim exploits conservation of particle number and the z component of spin, symmetries present in a wide range of fermionic systems, to dramatically reduce memory usage and simulation time compared to general-purpose quantum circuit simulators. Compared to FQE, a library with similar functionality, ffsim differs in software design and is faster on a representative set of simulation benchmarks. Beyond state vector evolution by basic fermionic gates, ffsim offers a number of additional features including variational ansatzes, Hamiltonian time evolution via Trotter-Suzuki product formulas, efficient sampling of Slater determinants, seamless integration with Qiskit and PySCF, and comprehensive documentation. We demonstrate ffsim's capabilities on scientific applications involving quantum circuits of up to 64 qubits.
Zenodo (CERN European Organization for Nuclear Research) · 2026-05-05
datasetOpen accessData for manuscript "Can phaseless auxiliary-field quantum Monte Carlo with broken symmetry trials describe iron-sulfur clusters?".
ArXiv.org · 2026-05-05
articleOpen accessSenior authorPhaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) has emerged as a promising electronic structure method for correlated electronic systems. However, the quality of its predictions depends critically on the choice of trial wavefunction, and it is not obvious how to make an optimal choice especially for strongly correlated states of large systems. Mean-field wavefunctions are compelling trial wavefunction candidates as they map directly to chemical concepts and can be obtained with $O(N^4)$ cost. Yet in the strongly correlated regime one faces a symmetry dilemma and the existence of multiple nearly-degenerate solutions. In this work we investigate active space models of [2Fe-2S]$^{2+}$, mixed-valent [4Fe-4S]$^{2+}$, and [4Fe-4S]$^{4+}$ and explore the sensitivity of ph-AFQMC to the choice of unrestricted Hartree-Fock trial wavefunction. We find that chemical properties and physical symmetries, rather than the variational energy, ought to guide the choice of mean-field trial for ph-AFQMC (or reference state for coupled cluster models), and show that surprisingly accurate ground-state energies for these systems can be obtained. However, in all cases we find a rapidly vanishing overlap between the stochastic wavefunction and the UHF trial, indicating that the trials are suboptimal importance functions. By analogy to a similar situation in the stretched helium dimer cation, we show how this sampling bias pushes ph-AFQMC towards artificially negative energies, which evidently can be compensated for by the phaseless bias in certain cases.
ffsim: Faster simulation of fermionic quantum circuits
arXiv (Cornell University) · 2026-05-04
preprintOpen accessWe present ffsim, an open-source software library for fast simulation of fermionic quantum circuits. ffsim exploits conservation of particle number and the z component of spin, symmetries present in a wide range of fermionic systems, to dramatically reduce memory usage and simulation time compared to general-purpose quantum circuit simulators. Compared to FQE, a library with similar functionality, ffsim differs in software design and is faster on a representative set of simulation benchmarks. Beyond state vector evolution by basic fermionic gates, ffsim offers a number of additional features including variational ansatzes, Hamiltonian time evolution via Trotter-Suzuki product formulas, efficient sampling of Slater determinants, seamless integration with Qiskit and PySCF, and comprehensive documentation. We demonstrate ffsim's capabilities on scientific applications involving quantum circuits of up to 64 qubits.
Journal of Chemical Theory and Computation · 2026-03-03
articleCorrespondingIn this study, we evaluate multiconfigurational trial wave function protocols for phaseless auxiliary field quantum Monte Carlo (ph-AFQMC) on transition metal containing systems. First, we benchmark vertical ionization potentials for 22 3d transition metal complexes against published high-accuracy ph-AFQMC values in a double-ζ basis set. We then compute the vertical ionization potential for a set of six metallocenes using our best-performing protocol, alongside ph-AFQMC using a configuration interaction singles and doubles (CISD) trial state. We also analyze the performance of canonical coupled-cluster theory with singles, doubles and perturbative triples (CCSD(T)), as well as its local approximation using domain-based local pair natural orbitals (DLPNO–CCSD(T1)) using different reference orbitals. To reach the complete-basis-set (CBS) limit, we examine several extrapolation schemes and report CBS-limit ph-AFQMC and CCSD(T) values alongside experimental results. We find that ph-AFQMC with the best-performing trial in a triple-ζ basis, followed by CBS correction from DLPNO–CCSD(T1) with unrestricted B3LYP reference orbitals, yields small deviations from experiment at modest cost. Using a CISD trial state in ph-AFQMC gives the closest agreement with experiment (errors <2 kcal/mol), albeit with lower scalability.
Recent grants
Frequent coauthors
- 172 shared
Richard A. Friesner
- 149 shared
Shiwei Zhang
Flatiron Institute
- 133 shared
David R. Reichman
Columbia University
- 79 shared
Pierre A. Devlaminck
Columbia University
- 79 shared
Dilek Coskun
Schrodinger (United States)
- 78 shared
John Weber
- 77 shared
Evan J. Arthur
William & Mary
- 74 shared
Luis M. Campos
Tecnológico de Monterrey
Labs
Awards & honors
- Ruth L. Kirschstein National Research Service Award from the…
- Louis Hammett Award for Excellence in Graduate Research from…
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