Quartic force fields (QFFs) have been shown to be one of the most effective ways to efficiently compute vibrational frequencies for small molecules. In this letter we discuss how the simple-internal or bond-length bond-angle (BLBA) coordinates can be transformed into Morse-cosine (-sine) coordinates which produce potential energy surfaces from QFFs that possess proper limiting behavior and can describe the vibrational (or rovibrational) energy levels of an arbitrary molecular system to 5 cm(-1) or better compared to experiment. We investigate parameter scaling in the Morse coordinate, symmetry considerations, and examples of transformed QFFs making use of the MULTIMODE, TROVE, and VTET variational vibrational methods. (c) 2013 Elsevier B.V. All rights reserved.