  Quantum computing is expected to play an important role in solving the
problem of huge computational costs in various applications by utilizing the
collective properties of quantum states, including superposition, interference,
and entanglement, to perform computations. Quantum mechanical (QM) methods are
candidates for various applications and can provide accurate absolute energy
calculations in structure-based methods. QM methods are powerful tools for
describing reaction pathways and their potential energy surfaces (PESs). In
this study, we applied quantum computing to describe the PES of the bimolecular
nucleophilic substitution (SN2) reaction between chloromethane and chloride
ions. We performed noiseless and noise simulations using quantum algorithms and
compared the accuracy and noise effects of the ansatzes. In noiseless
simulations, the results from UCCSD and k-UpCCGSD are similar to those of full
configurational interaction (FCI) with the same active space, which indicates
that quantum algorithms can describe the PES of the SN2 reaction. In noise
simulations, UCCSD is more susceptible to quantum noise than k-UpCCGSD.
Therefore, k-UpCCGSD can serve as an alternative to UCCSD to reduce quantum
noisy effects in the noisy intermediate-scale quantum era, and k-UpCCGSD is
sufficient to describe the PES of the SN2 reaction in this work. The results
showed the applicability of quantum computing to the SN2 reaction pathway and
provided valuable information for structure-based molecular simulations with
quantum computing.
