<p></p><p>Very
often in order to understand physical and chemical processes taking place among
several phases fractionation of naturally abundant isotopes is monitored. Its measurement can
be accompanied by theoretical determination to provide a more insightful interpretation of observed
phenomena. Predictions are challenging due to the complexity of the effects involved in fractionation such as solvent
effects and non-covalent interactions governing the behavior of the system
which results in the necessity of using large models of those systems. This is
sometimes a bottleneck and limits the theoretical description to only a few methods.<br>
In this work vapour pressure isotope effects on evaporation from various
organic solvents (ethanol, bromobenzene, dibromomethane, and trichloromethane)
in the pure phase are estimated by combining force field or self-consistent charge density-functional tight-binding (SCC-DFTB)
atomistic simulations with path integral principle. Furthermore, the recently developed Suzuki-Chin path
integral is tested. In general, isotope effects are predicted qualitatively for most of the cases, however, the
distinction between position-specific isotope effects observed for ethanol was only reproduced by SCC-DFTB, which
indicates the importance of using non-harmonic bond approximations.<br>
Energy decomposition analysis performed using the symmetry-adapted perturbation
theory (SAPT) revealed sometimes quite substantial differences in interaction energy depending on
whether the studied system was treated classically or quantum mechanically. Those observed differences might
be the source of different magnitudes of isotope effects predicted using these two different levels of
theory which is of special importance for the systems governed by non-covalent interactions.</p><br><p></p>
Small matrix disentanglement of the path integral: Overcoming the exponential tensor scaling with memory length
