Exploring the promise and limitations of point-charge-free potentials for hydrocarbon modeling

Models for hydrated and neat hydrocarbons are developed with the adaptive force matching method using either B3LYP-D3(BJ) or MP2 as reference. Hydrocarbons, being non-polar, present an opportunity to explore the possibility of creating force fields without modeling electrostatics. For liquid methane, ethane, cyclopentane, and cyclohexene, models without partial charges achieved excellent agreement with experiments when fitted against MP2 reference with SAPT dispersion. In aqueous solutions, models without solute partial charges maintain high quality for saturated hydrocarbons but exhibit reduced accuracy for cyclohexene. Notably, liquid methane's weak intermolecular interactions make it particularly sensitive to the choice of reference method. Our findings indicate that MP2 with the aug-cc-pVQZ basis set achieves the best overall agreement with experimental results.