Optical electronic absorption spectroscopy and corresponding selection rules have played a promin... more Optical electronic absorption spectroscopy and corresponding selection rules have played a prominent role in the development of the theory of electronic structure of materials. In this work, we expand the modern toolbox of chemistry and materials science to include electron and ion microscopies and spectroscopies, allowing spatially resolved interrogation of materials atomic and electronic structures by beams of charged particles. Specifically, we formulate and discuss the selection rules for electronic excitation due to the interaction between materials and beams of charged particles. We show that transition probabilities for point charge induced electronic excitations depend strongly on the position of the external charged particles, and can significantly deviate from those derived from the electric dipole (long-wavelength) approximation. We present and implement expressions within the linear response TD-DFT framework for rates of transition between the ground and excited states induced by an external point charge. The point charge induced transition rates for particular electronic excitations from linear response TD-DFT were validated through comparison to excited state populations from real time TD-DFT simulations following an impulsive point charge perturbation, then evaluated on a three-dimensional grid to map their spatial dependence for a small polybenzoid. This method, when combined with information about excited state energy gradients, represents a first step toward an ab initio framework for probing the structural response of materials under irradiation by charged particles due to inelastic scattering. Engineering electron beam interaction with matter to manipulate single atoms and localized electronic states offers a revolutionary new regime beyond
Understanding complex chemical systems-such as biomolecules, catalysts, and novel materials-is a ... more Understanding complex chemical systems-such as biomolecules, catalysts, and novel materials-is a central goal of quantum simulations. Near-term strategies hinge on the use of variational quantum eigensolver (VQE) algorithms combined with a suitable ansatz. However, straightforward application of many chemically-inspired ansatze yields prohibitively deep circuits. In this work, we employ several circuit optimization methods tailored for trapped-ion quantum devices to enhance the feasibility of intricate chemical simulations. The techniques aim to lessen the depth of the unitary coupled cluster with singles and doubles (uCCSD) ansatz's circuit compilation, a considerable challenge on current noisy quantum devices. Furthermore, we use symmetry-inspired classical post-selection methods to further refine the outcomes and minimize errors in energy measurements, without adding quantum overhead. Our strategies encompass optimal mapping from orbital to qubit, term reordering to minimize entangling gates, and the exploitation of molecular spin and point group symmetry to eliminate redundant parameters. The inclusion of error mitigation via post-selection based on known molecular symmetries improves the results to near milli-Hartree accuracy. These methods, when applied to a benzene molecule simulation, enabled the construction of an 8-qubit circuit with 69 two-qubit entangling operations, pushing the limits for variational quantum eigensolver (VQE) circuits executed on quantum hardware to date. 1
We introduce QuantumGEP , a scientific computer program that uses gene expression programming (GE... more We introduce QuantumGEP , a scientific computer program that uses gene expression programming (GEP) to find a quantum circuit that either (i) maps a given set of input states to a given set of output states, or (ii) transforms a fixed initial state to minimize a given physical quantity of the output state. QuantumGEP is a driver program that uses evendim , a generic computational engine for GEP, both of which are free and open source. We apply QuantumGEP as a powerful solver for MaxCut in graphs, and for condensed matter quantum many-body Hamiltonians.
ARTICLES-Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry-Comparative study of He3, Ne3, and Ar3 using hyperspherical coordinates
Time-dependent density functional theory (TD-DFT) is nowadays routinely applied to molecular and ... more Time-dependent density functional theory (TD-DFT) is nowadays routinely applied to molecular and nanoscaled condensed-phase materials for the calculation of electronic excitation energies and their associated optical transition probabilities. In this paper, we derive and implement expressions within the linear response TD-DFT framework for rates of transition between the ground and excited states induced by an external point charge. Symmetry considerations are given for the coupling between electronic states of well defined parity in two extreme limits of the point charge's position, and a general method to determine the range of point charge positions over which electric dipole selection rules hold for describing a given point charge induced electronic excitation is presented. The point charge induced transition rates for particular electronic excitations from linear response TD-DFT were validated through comparison to excited state populations from real time TD-DFT simulations following an impulsive 1 point charge perturbation, then evaluated on a three-dimensional grid to map their spatial dependence for a small polybenzoid. This method, when combined with information about excited state energy gradients, represents a first step toward an ab initio framework for probing the structural response of materials under electron beam irradiation due to inelastic scattering.
Journal of the American Chemical Society, Nov 27, 2003
The (TCNE)2 2dimer dianion formed by connecting two TCNEanions via a four-center, twoelectron π-o... more The (TCNE)2 2dimer dianion formed by connecting two TCNEanions via a four-center, twoelectron π-orbital bond is studied using ab initio theoretical methods and a model designed to simulate the stabilization due to surrounding counterions. (TCNE)2 2is examined as an isolated species and in a solvation environment representative of tetrahydrofuran (THF) solvent. The intrinsic strength of this novel bond and the influences of internal Coulomb repulsions, of solvent stabilization and screening, and of counterion stabilization are all considered. The geometry, electronic and thermodynamic stabilities, electronic absorption spectra, and electron detachment energies of this novel dianion are examined to help understand recent experimental findings. Our findings lead us to conclude that the (TCNE) 2 2dianion's observation in solid materials is likely a result of its stabilization by surrounding countercations. Moreover, our results suggest the dianion is geometrically metastable in THF solution, with a barrier to dissociation into two TCNEanions that can be quickly surmounted at room temperature but not at 77 K. This finding is consistent with what is observed in laboratory studies of low-and room-temperature solutions of salts containing this dianion. Finally, we assign two peaks observed (at 77 K in methyl-THF glass) in the UV-vis region to (1) electronic transitions involving the four-center orbitals and (2) detachment of an electron from the four-center π-bonding orbital to generate (TCNE) 2-+ e- .
We introduce QuantumGEP, a scientific computer program that uses gene expression programming (GEP... more We introduce QuantumGEP, a scientific computer program that uses gene expression programming (GEP) to find a quantum circuit that either (i) maps a given set of input states to a given set of output states, or (ii) transforms a fixed initial state to minimize a given physical quantity of the output state. QuantumGEP is a driver program that uses evendim, a generic computational engine for GEP, both of which are free and open source. We apply QuantumGEP as a powerful solver for MaxCut in graphs, and for condensed matter quantum many-body Hamiltonians.
We have introduced a computational methodology to study vibrational spectroscopy in clusters incl... more We have introduced a computational methodology to study vibrational spectroscopy in clusters inclusive of critical nuclear quantum effects. This approach is based on the recently developed quantum wavepacket ab initio molecular dynamics method that combines quantum wavepacket dynamics with ab initio molecular dynamics. The computational efficiency of the dynamical procedure is drastically improved (by several orders of magnitude) through the utilization of wavelet-based techniques combined with the previously introduced time-dependent deterministic sampling procedure measure to achieve stable, picosecond length, quantumclassical dynamics of electrons and nuclei in clusters. The dynamical information is employed to construct a novel cumulative flux/velocity correlation function, where the wavepacket flux from the quantized particle is combined with classical nuclear velocities to obtain the vibrational density of states. The approach is demonstrated by computing the vibrational density of states of [Cl-H-Cl]-, inclusive of critical quantum nuclear effects, and our results are in good agreement with experiment. A general hierarchical procedure is also provided, based on electronic structure harmonic frequencies, classical ab initio molecular dynamics, computation of nuclear quantum-mechanical eigenstates, and employing quantum wavepacket ab initio dynamics to understand vibrational spectroscopy in hydrogen-bonded clusters that display large degrees of anharmonicities.
The nuclear quantum effects on the zero-point energy (ZPE), influencing adsorption of H 2 and iso... more The nuclear quantum effects on the zero-point energy (ZPE), influencing adsorption of H 2 and isotopologues on metal ions, are examined using normal mode analysis of ab initio electronic structure results for complexes with 17 metal cations. The lightest metallic nuclei, Li and Be, are found to be the most 'quantum'. The largest selectivity in adsorption is predicted for Cu, Ni and Co ions. Analysis of the nuclear wavepacket dynamics on the ground state electronic potential energy surfaces (PES) performed for complexes of Li + and Cu +2 with H 2 /D 2 /HD. shows that the PES anharmonicity changes the ZPE by up to 9%.
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Papers by Jacek Jakowski