Less than 5% of the human genome codes for protein, but about 90% codes for transcribed RNA. Structures and functions for much of this RNA are not known. Moreover, it appears to be more difficult to determine and to predict structures of RNAs than structures of proteins. For proteins and RNA, 3D predictions are made by both "knowledge based" and "quantum mechanical based" force fields. NMR experiments on simple RNA systems can provide benchmarks for testing methods to predict 3D structure. Results from NMR experiments and molecular dynamics simulations will be presented for single stranded, unpaired tetramers and for base paired structures of RNA. Comparisons reveal strengths and weaknesses of force fields.