We present a hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics study of 18-crown-6 (18c6) in both polar hydrogen bonding and nonpolar solvents. The QM/MM method described here is based on our previous QM/MM study of K+/18c6(1) which employed the semiempirical AM1 method for 18c6 and the SPC/e model for H2O. We improve our previous QM/MM method by including MM torsion terms to better describe the energetics of OCCO and COCC rotamers. The torsion terms are parametrized against the ab initio calculations of Jaffe et al. for 1,2-dimethoxyethane.(2,3) The resulting torsion-modified AM1 (TAMI) Hamiltonian is used with our previous QM/MM parameters to describe the conformational energetics of free 18c6 in both H2O and CCl4. The TAM1 method describes optimized geometries and relative energetics for C-i and D-3d configurations of 18c6 slightly better than does the unmodified AM1 method. TAM1 simulation results of OCCO and COCC torsion angle distributions show 18c6 maintains a preorganized oxygen-lined cavity in H2O, in contrast to the simulation in CCl4. The crown ether strongly binds two bridging H2O molecules that help maintain the 18c6 cavity in H2O. These two waters, together, have a simulation average 18c6/H2O stabilization energy of -23.8 kcal/mol. These results contrast markedly with an 18c6/H2O simulation using the unmodified AM1 method. The total QM/MM 18c6/H2O interaction energy is -82.6 kcal/mol and -63.7 kcal/mol for the TAM1 and AM1 simulations, respectively. The AM1 18c6/H2O simulation reveals that 18c6 lacks both a preorganized cavity and the two bound waters. Without correction for rotamer energetics, the AM1 method is not useful for simulations of flexible systems like 18c6. However, semiempirical QM/MM methods, with corrections for torsions, are perhaps preferable to both pairwise-additive and polarizable-empirical force field methods, since the QM treats electronic polarization as a natural part of the simulation.