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
their equilibrium geometry, and thus, the construction of multiconfigurational
wave functions without performing post-HF calculations on classical
computers is highly desired for applying the method to a wide variety
of chemistries and physics. In this work, we propose a method to construct
multiconfigurational initial guess wave functions suitable for QPEA-based
full-CI calculations on quantum computers, by utilizing diradical
characters computed from spin-projected UHF wave functions. The proposed
approach drastically improves the wave function overlap, particularly
in molecules with intermediate diradical characters.
Mesityl derivatives of the unknown dibenzopentalene isomer dibenzo[a,f]pentalene were synthesized. The molecular geometry and physical properties of dibenzo[a,f]pentalene were investigated. Dibenzo[a,f]pentalene combines a large antiaromatic and appreciable singlet open-shell character, properties not shared by well-known isomer dibenzo[a,e]pentalene.
NMR spectroscopy is a powerful tool
to investigate molecular structure
and dynamics. The poor sensitivity of this technique, however, limits
its ability to tackle questions requiring dilute samples. Low-concentration
photochemically induced dynamic nuclear polarization (LC-photo-CIDNP)
is an optically enhanced NMR technology capable of addressing the
above challenge by increasing the detection limit of aromatic amino
acids in solution up to 1000-fold, either in isolation or within proteins.
Here, we show that the absence of NMR-active nuclei close to a magnetically
active site of interest (e.g., the structurally diagnostic 1Hα–13Cα pair
of amino acids) is expected to significantly increase LC-photo-CIDNP
hyperpolarization. Then, we exploit the spin-diluted tryptophan isotopolog
Trp-α-13C-β,β,2,4,5,6,7-d7 and take advantage of the above prediction to experimentally achieve
a ca 4-fold enhancement in NMR sensitivity over regular LC-photo-CIDNP.
This advance enables the rapid (within seconds) detection of 20 nM
concentrations or the molecule of interest, corresponding to a remarkable
3 ng detection limit. Finally, the above Trp isotopolog is amenable
to incorporation within proteins and is readily detectable at a 1
μM concentration in complex cell-like media, including Escherichia coli cell-free extracts.
Quantum computers are capable to efficiently perform full configuration interaction (FCI) calculations of atoms and molecules by using the quantum phase estimation (QPE) algorithm. Because the success probability of the QPE depends on the overlap between approximate and exact wave functions, efficient methods to prepare accurate initial guess wave functions enough to have sufficiently large overlap with the exact ones are highly desired. Here, we propose a quantum algorithm to construct the wave function consisting of one configuration state function, which is suitable for the initial guess wave function in QPE-based FCI calculations of open-shell molecules, based on the addition theorem of angular momentum. The proposed quantum algorithm enables us to prepare the wave function consisting of an exponential number of Slater determinants only by a polynomial number of quantum operations.
Zero-field splitting (ZFS) tensors (D tensors) of organic high-spin oligonitrenes/oligocarbenes up to spin-septet are quantitatively determined on the basis of quantum chemical calculations. The spin-orbit contributions, D(SO) tensors are calculated in terms of a hybrid CASSCF/MRMP2 approach, which was recently proposed by us. The spin-spin counterparts, D(SS) tensors are computed based on McWeeny-Mizuno's equation in conjunction with the RODFT spin densities. The present calculations show that more than 10% of ZFS arises from spin-orbit interactions in the high-spin nitrenes under study. Contributions of spin-bearing site-site interactions are estimated with the aid of a semi-empirical model for the D tensors and found to be ca. 5% of the D(SO) tensor. The analysis of intermediate states reveal that the largest contributions to the calculated D(SO) tensors are attributed to intra-site spin flip excitations and delocalized π and π* orbitals play an important role in the inter-site spin-orbit interactions.
A quantum algorithm “Bayesian exchange coupling parameter calculator with broken-symmetry wave function (BxB)” enables us to calculate Heisenberg exchange coupling parameter J without inspecting total energies of individual spin states, within 1 kcal mol−1 of energy tolerance.
A molecular spin quantum computer (MSQC) requires electron spin qubits, which pulse-based electron spin/magnetic resonance (ESR/MR) techniques can afford to manipulate for implementing quantum gate operations in open shell molecular entities. Importantly, nuclear spins, which are topologically connected, particularly in organic molecular spin systems, are client qubits, while electron spins play a role of bus qubits. Here, we introduce the implementation for an adiabatic quantum algorithm, suggesting the possible utilization of molecular spins with optimized spin structures for MSQCs. We exemplify the utilization of an adiabatic factorization problem of 21, compared with the corresponding nuclear magnetic resonance (NMR) case. Two molecular spins are selected: one is a molecular spin composed of three exchange-coupled electrons as electron-only qubits and the other an electron-bus qubit with two client nuclear spin qubits. Their electronic spin structures are well characterized in terms of the quantum mechanical behaviour in the spin Hamiltonian. The implementation of adiabatic quantum computing/computation (AQC) has, for the first time, been achieved by establishing ESR/MR pulse sequences for effective spin Hamiltonians in a fully controlled manner of spin manipulation. The conquered pulse sequences have been compared with the NMR experiments and shown much faster CPU times corresponding to the interaction strength between the spins. Significant differences are shown in rotational operations and pulse intervals for ESR/MR operations. As a result, we suggest the advantages and possible utilization of the time-evolution based AQC approach for molecular spin quantum computers and molecular spin quantum simulators underlain by sophisticated ESR/MR pulsed spin technology.
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