Molecular coordinates for instantaneous normal mode calculations. I. Coordinate dependence

We demonstrate that the results of instantaneous normal mode (INM) calculations depend strongly on the coordinate system used, particularly when flexible molecules are employed. Appropriate INM treatments of rotation, bending, and rotational kinetic energy are illustrated and discussed, and a set of criteria for the selection of coordinates for INM calculations on molecular liquids is presented. A general scheme for developing molecular coordinates is introduced. This method simplifies the derivation of the expressions required for INM calculations, particularly for the kinetic energy. A technique for correcting the INM frequencies, applicable in some situations, is also presented.