Second-derivative coupling matrix molecular systems

Normal modes are useful because they correspond to collective motions of the atoms in a coupled system that can be individually excited. The three normal modes of water are schematically illustrated in Figure 5.15 a non-linear molecule with N atoms has 3N — 6 normal modes. The frequencies of the normal modes together with the displacements of the individual atoms may be calculated from a molecular mechanics force field or from the wavefunction using the Hessian matrix of second derivatives Of course, if we... 

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