Quantum locality and equilibrium properties in low-temperature parahydrogen: A multiscale simulation study

Parahydrogen is the spin-zero singlet state of molecular hydrogen, which at low temperature (between 14 and 25 K) is in a fluid state. A classical treatment of the system leads to unphysical freezing, and the inclusion of quantum delocalization of the molecule is then required to obtain a realistic description of its equilibrium properties. In the present work, we employ the classical-quantum adaptive resolution method AdResS to investigate the spatial extension of quantum delocalization effects in the bulk fluid at low temperature. Specifically, we simulate a small, spherical region of the system in full quantum detail: this region is coupled to a bulk of coarse-grained particles with classical, quantum-derived effective interactions obtained from quantum simulations. The two regions are interfaced through open boundaries and in conditions of thermodynamic equilibrium. Structural properties of the fluid, namely, pair distribution functions, are measured for different sizes of the quantum region. The results of this work show that, for the thermodynamic conditions corresponding to the range of temperature between 14 and 25 K, the bead-based, quantum structural properties of low-temperature parahydrogen are deemed local and do not require the support of an explicit quantum bulk.