Phys. Chem. Chem. Phys., Published online (2021) (DOI: 10.1039/d1cp03156b) Open access Quantum computers can perform full configuration interaction (full-CI) calculations by utilising the quantum phase estimation (QPE) algorithms including Bayesian phase estimation (BPE) and iterative quantum phase estimation (IQPE). In these quantum algorithms, the time evolution of wave functions for atoms and molecules is simulated conditionally with an ancillary qubit as the control, which make implementation to real quantum devices difficult. Also, most of the problems in chemistry discuss energy differences between two electronic states rather than total energies themselves, and thus direct calculations of energy gaps are promising for future applications of quantum computers to real chemistry problems. In the […]

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## Valence tautomerism in a [2 × 2] Co4 grid complex containing a ditopic arylazo ligand

Chem. Commun., 57, pp.6213-6216 (2021) (DOI: 10.1039/D1CC01991K) We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

## A quantum algorithm for the direct calculations of vertical ionization energies.

J. Phys. Chem. Lett. 2021, #, 2880–2885, Published online (DOI: 10.1021/acs.jpclett.1c00283) Open access Recently, a quantum algorithm that is capable of directly calculating the energy gap between two electronic states having different spin quantum numbers without inspecting the total energy of the individual electronic states was proposed. This quantum algorithm guarantees an exponential speedup, like quantum phase estimation (QPE)-based full-CI, with much lower costs. In this work, we propose a modified quantum circuit for the direct calculations of spin state energy gaps to reduce the number of qubits and quantum gates, extending the quantum algorithm to the direct calculation of vertical ionization energies. Numerical quantum circuit simulations for the ionization of […]

## A quantum algorithm for spin chemistry: a Bayesian exchange coupling parameter calculator with broken-symmetry wave functions.

Chem. Sci. 2021, , Published online (DOI: 10.1039/d0sc04847j) Open access The Heisenberg exchange coupling parameter J (H = −2JSi · Sj) characterises the isotropic magnetic interaction between unpaired electrons, and it is one of the most important spin Hamiltonian parameters of multi-spin open shell systems. The J value is related to the energy difference between high-spin and low-spin states, and thus computing the energies of individual spin states are necessary to obtain the J values from quantum chemical calculations. Here, we propose a quantum algorithm, ayesian echange coupling parameter calculator with roken-symmetry wave functions (BxB), which is capable of computing the J value directly, without calculating the energies of individual spin states. The BxB algorithm is composed of the quantum simulations of the time […]

## [2020 PCCP HOT Article] A probabilistic spin annihilation method for quantum chemical calculations on quantum computers

Phys. Chem. Chem. Phys. 2020, 22, 20990-20994 (DOI: 10.1039/D0CP03745A) Open access [ 2020 PCCP HOT Article ] A probabilistic spin annihilation method based on the quantum phase estimation algorithm is presented for quantum chemical calculations on quantum computers. This approach can eliminate more than one spin component from the spin contaminated wave functions by single operation. Comparison with the spin annihilation operation on classical computers is given. 大学の新着ニュースに掲載されました。https://www.osaka-cu.ac.jp/ja/news/2020/201012-2

## Trityl-Aryl-Nitroxide-Based Genuinely gEngineered Biradicals, As Studied by Dynamic Nuclear Polarization Multifrequency ESR/ENDOR, Arbitrary Wave Generator Pulse Microwave Waveform Spectroscopy, and Quantum Chemical Calculations

J. Phys. Chem. A 2019, 123, 7507-7517. (DOI: 10.1021/acs.jpca.9b07169) Trityl and nitroxide radicals are connected by π-topologically controlled aryl linkers, generating genuinely g-engineered biradicals. They serve as a typical model for biradicals in which the exchange (J) and hyperfine interactions compete with the g-difference electronic Zeeman interactions. The magnetic properties underlying the biradical spin Hamiltonian for solution, including J’s, have been determined by multifrequency CW-ESR and 1H ENDOR spectroscopy and compared with those obtained by quantum chemical calculations. The experimental J values were in good agreement with the quantum chemical calculations. The g-engineered biradicals have been tested as a prototype for AWG (Arbitrary Wave Generator)-based spin manipulation techniques, which enable GRAPE […]

## [ 2019 PCCP HOT Article ]Quantum chemistry on quantum computers: quantum simulations of the time evolution of wave functions under the S^{2} operator and determination of the spin quantum number *S*

Phys. Chem. CHem. Phys. 2019, 21, 15356-15361. (DOI: 10.1039/C9CP02546D) Open access [ 2019 PCCP HOT Article ] Quantum computers have an enormous impact on quantum chemical calculations. Approaches to calculate the energies of atoms and molecules on quantum computers by utilizing quantum phase estimation (QPE) and the variational quantum eigensolver (VQE) have been well documented, and dozens of methodological improvements to decrease computational costs and to mitigate errors have been reported until recently. However, the possible methodological implementation of observables on quantum computers such as calculating the spin quantum numbers of arbitrary wave functions, which is a crucial issue in quantum chemistry, has been discussed less. Here, we propose a quantum […]

## The 6th Awaji International Workshop on “Electron Spin Science & Technology: Biological and Materials Science Oriented Applications” (AWEST 2019)

## Quantum Chemistry on Quantum Computers: A Method for Preparation of Multiconfigurational Wave Functions on Quantum Computers without Performing Post-Hartree–Fock Calculations

ACS Cent. Sci. 2019, 5, pp.167-175 (DOI: 10.1021/acscentsci.8b00788) Open access The full configuration interaction (full-CI) method is capable of providing the numerically best wave functions and energies of atoms and molecules within basis sets being used, although it is intractable for classical computers. Quantum computers can perform full-CI calculations in polynomial time against the system size by adopting a quantum phase estimation algorithm (QPEA). In the QPEA, the preparation of initial guess wave functions having sufficiently large overlap with the exact wave function is recommended. The Hartree–Fock (HF) wave function is a good initial guess only for closed shell singlet molecules and high-spin molecules carrying no spin-β unpaired electrons, around […]

## Open shell electronic state calculations on quantum computers: A quantum circuit for the preparation of configuration state functions based on Serber construction

Chemical Physics Letters: X 2019, 1, 10002. (DOI: 10.1016/j.cpletx.2018.100002) Open access Full configuration interaction (full-CI) calculations can be executed efficiently on quantum computers (QCs) by utilizing a quantum phase estimation algorithm (QPEA). In the QPEA-based full-CI on QCs, the preparation of the initial guess wave functions having large overlap with the full-CI root is crucial. Recently, we proposed a quantum circuit to prepare spin symmetry-adapted configuration state functions (CSFs) toward open shell electronic structure calculations of molecules on QCs. Here, we propose an improved quantum circuit, based on Serber construction of spin eigenfunctions, enabling us to prepare CSFs with much less and easier-to-implement quantum gates than the previously proposed one. […]