The successful validation of the JOYCE3.0 procedure in handling both rigid molecules with "exotic" shapes and long, semi-flexible chains prompted us to complete these first three benchmarks with a highly flexible compound, i.e. target III (see Figure ). n-tetradecane is indeed a long and very flexible linear alkane, characterized by a chain of fourteen Carbon atoms and several rotatable dihedrals, which play a crucial role in determining the overall conformation of the molecule. Moreover, target III it undergo significant backbone distortion even under ambient conditions, resulting in head-to-tail or similar contacts during its conformational dy-namics. Therefore, as for target molecule II, n-tetradecane was parameterized through the new JOYCE3.0 two-step procedure using both the QM Hessian matrix and the relaxed scans along the flexible dihedrals, yet introducing the use of internal Lennard-Jones (LJ) interactions, as described in equations ( ) and (11). As detailed in Section S5.3 of the Supporting Information, target III parameterization was carried out on an ensamble of 252 ICs, which comprises both stiff (bond stretching and angle bending) and soft (flexible dihedrals and 1-8 or further intramolecular interactions) ICs. This large set can be reduced to 19 independent force constants exploiting n-tetradecane symmetry (see also Figure ), obtaining a final standard deviation of 0.3 kJ/mol, which confirms that the QMD-FF is able to accurately represent the QM PES both in a LHA and along the relaxed profiles of the scanned dihedrals (Figures and). Nonetheless, to achieve a more comprehensive validation, which also accounts for possible coupling among ICs during collective displacements, we also repeated some of the validation tests based on BOMD, previously discussed for target I. was merged with the OPLS intermolecular parameters, allowing for simulation of systems with large numbers of interacting tetradecane polymers. For a final evaluation of the resulting FF, as reported in Table , selected thermodynamic properties, averaged for 100 ns at 1 atm and 298 K on a system of 1000 n-tetradecane molecules in of condensed phase, were compared with those obtained with the same protocol, yet using two transferable FFs, namely GAFF and OPLS. The performances delivered by the three FFs are quite different: while OPLS and our QMD-FF (which share the same intermolecular term) yield an overall good agreement with the experimental measures, GAFF simulates a too dense phase, with a significant overestimation of the FF ρ (kg/m 3 ) ∆H vap (kJ/mol) c P ( J mol -1 K) η (mPa s) GAFF vaporization enthalpy, suggesting too strong intermolecular interactions. Indeed, as revealed from Figure in the Supporting Information, evident crystalline sub-domains start forming already at 298 K, confirming the GAFF deficiency in well representing long alkyl chains. In particular, as displayed in the c) and d) panels of Figure , the overestimated interaction in GAFF alters the molecular conformation artificially favoring the all-anti conformers, hence tetradecane conformational dynamics is further confirmed in Table , where the distribution of the chain conformers, averaged along the 100 ns dynamics were collected for the three models, from which interesting information can be retrieved also for the comparison between OPLS and the QMD-FF. In fact, despite they share the same inter-molecular term, the two FFs do not deliver identical thermodynamic properties, the QMD-FF being closer to the experiment in all descriptors, except viscosity. The increased accuracy can be traced back only to the intramolecular re-parametrization, which allows the alkyl chains to follow a different (see Table ) and more realistic conformational dynamics, leading to a better estimate of the molecular excluded volume and, in turn, of the intermolecular interaction energy.