There is always a concern about the presence of "holes" in the fits, i.e., regions where the energy is predicted to be very negative, relative to the energy of the GM. It is inevitable, particularly with high-dimensionality surfaces, that there will be places where the interpolation or extrapolation of the surface will fail in regions which lack data. This can also occur by over fitting, which happens even in our over-determined least-squares approach when ratio of data points to coefficients approaches factors smaller than about 10 for C 14 H 30 . In these cases, the data may be precisely fit, but the regions between points or the regions outside the data set are not well fit. The higher the number of coefficients, the more oscillatory will be the fit, and the more likely it will be that negative energies will be encountered. Of course, this causes serious problems for any type of molecular dynamics.