Tsutomu Kawatsu

Efficient algorithms for semiclassical instanton calculations based on discretized path integrals

Path integral instanton method is a promising way to calculate the tunneling splitting of energies for degenerated two state systems. In order to calculate the tunneling splitting, we need to take the zero temperature limit, or the limit of infinite imaginary time duration. In the method developed by Richardson and Althorpe [J. Chem. Phys. 134, 054109 (2011)], the limit is simply replaced by the sufficiently long imaginary time. In the present study, we have developed a new formula of the tunneling splitting based on the discretized path integrals to take the limit analytically. We have applied our new formula to model systems, and found that this approach can significantly reduce the computational cost and gain the numerical accuracy. We then developed the method combined with the electronic structure calculations to obtain the accurate interatomic potential on the fly. We present an application of our ab initio instanton method to the ammonia umbrella flip motion.

An efficient computational method for the implementation of a semi-classical instanton approach using discretized path integrals

In the present paper, a numerical method for a semi-classical instanton method was examined. We implemented the instanton approach using discretized path integrals. The computational accuracy of the method is controlled by the following two parameters: the imaginary time duration (τ) and the time increment (Δτ), which represents the discretized path integral. To obtain accurate results, a long τ must be used in combination with a short Δτ; however, because the computational cost is virtually proportional to τ/Δτ, the instanton calculations were computationally expensive under these conditions. In the present study, we propose a method that reduces the computational cost and represents long τ instanton trajectories by employing an extended instanton trajectory from calculations based on a short τ. We applied the method to calculate tunnel splitting in a HO2 hydrogen transfer reaction using the double many-body extension IV potential as a validation.

A 3D-RISM integral equation study of a hydrated dipeptide

In this study, thermodynamic and molecular properties of a hydrated alanine dipeptide have been studied by the three-dimensional reference interaction site method (3D-RISM) theory. The excess chemical potential regarding hydration is calculated as a function of the dihedral angles and . The calculated results are found to be in reasonable agreement with extended ensemble simulation results. Partial molar volume is also calculated for each major structural state of the dipeptide. Overall trend is well described by the 3D-RISM calculations in comparison with molecular simulation and other integral equation results.

The isotope effects on a hydrogen transfer using path integral instanton method

In our previous work, we have developed an algorithm for calculating the tunnelling splittings using path integral instanton method with the discretised imaginary time approach. Our algorithm has enabled us to take a zero temperature limit, efficiently and accurately. In this paper, we investigate the effect of isotopomers on the tunnelling splittings and semiclassical tunnelling paths of a hydrogen transfer in a model molecular system HO2.