Development of a polarizable force field for acetonitrile based on a single-center multipole expansion

A new polarizable potential for describing the interaction between acetonitrile molecules is introduced. The electrostatic interaction is described using a single center multipole expansion (SCME) as in the original rigid SCME potential for H2O [Wikfeldt et al, PCCP 15, 2013 (16542)]. The electrostatic interaction includes multipole moments from dipole to hexadecapole, as well as anisotropic dipole-dipole, dipole-quadrupole and quadrupole-quadrupole polarizability tensors. The SCME model is free of point charges. The non-electrostatic part is described in a pair-wise fashion using a Born-Mayer repulsion potential together with damped dispersion interaction. The potential function is parametrized to fit the interaction energy of small (CH3CN)n, n=2-6, clusters calculated using the PBE0 hybrid functional with the many-body dispersion energy method (MBD@rsSCS). The parametrized potential function was then validated by calculating the α-phase of acetonitrile crystal, and the results found to agree well with PBE0-MBD@rSCS results, both for the cohesive energy and the bulk modulus.