• Dibyendu Mondal

      Articles written in Journal of Chemical Sciences

    • Quantum chemical investigation of thermochemistry in Calvin cycle

      Dibyendu Mondal Tumpa Sadhukhan Iqbal A Latif Sambhu N Datta

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      This work aims to verify the experimental thermochemistry of the reactions involved in Calvin cycle that produces glucose equivalent by using products from the light-activated reactions in chloroplast. The molecular geometry of each involved species in water has been optimized by density functional theory using SCRF-PCM methodology at M06-2X/6-311++G(3df,3pd) level. The thermal correction to Gibbs free energy of each solute has been calculated at the same level of theory. An explicit accounting of the intramolecular and intermolecular hydrogen bonding has been made for each solute molecule by using theoretically determined values from different sources. These data have been added together to obtain the standard Gibbs free energy 𝐺 for each molecule in solution. Finally, the free energy change 𝛥𝐺 of each involved reaction has been determined using the experimental concentrations under physiological conditions. The calculated 𝛥𝐺 values are generally in good agreement with the experimentally found free energy changes, with only a few relatively large deviations. Five regulating steps with moderately large and negative 𝛥𝐺 have been identified, whereas only three of them were clearly identified from experiment. We particularly show that the steps involving the formation of G3P from 3-PG and the regeneration of RuBP from Ru5P are thermodynamically strongly favored, and therefore, they take part in driving the metabolic process. We have illustrated Calvin cycle by vividly distinguishing the controlling steps from the potentially reversible reactions.

    • Corrections beyond coupled cluster singles and doubles through selected generalized rank-two operators: digital quantum simulation of strongly correlated systems


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      The hybrid quantum-classical variational quantum eigensolver (VQE) with the unitary coupled cluster (UCC) ansatz has been the method of choice for the digital quantum simulation of molecular energeticsin near-term noisy quantum devices. The most commonly used UCC ansatz with the single and double excitations (UCCSD) fails to achieve chemical accuracy when molecular strong correlation comes intopicture. In this exploratory work, we show that the UCCSD ansatz, complemented with a selected class of generalized excitation operators, taken in a partially factorized (disentangled) form, can encapsulate arbitrary high rank correlation. The choice of the class of generalized operators dictates the gate count, the upper bound of which is determined by UCCSD. We provide numerical examples with a few specific operator classes that show significant improvement in accuracy in energy over UCCSD without increase in the implementation cost.

      Development of a class of low depth factorized ansatze is discussed in the variational quantum eigensolver framework and their performance is assessed for systems with strong correlation.

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