Abstract
A general expression of wave vector dependent dynamical matrices for calculating the harmonic normal frequencies of molecular crystals, including polymer crystals, has been presented, along with the method of factorization according to the irreducible representation of the space group. The first derivative of the atom–atom potential with respect to interatomic distance, which was ignored in the commonly used central force approximation, has been included in the intermolecular force field. The method has been applied to the case of the orthorhombic and triclinic modifications of polyethylene. It has been indicated that the effect of inclusion of the linear terms on the normal frequencies is remarkable for the librational lattice modes (it lowers the frequencies by as much as 20 cm−1), while it is not drastic for the translational lattice modes as well as for the correlation splittings of the internal modes. Agreement between the observed and calculated frequencies of the lattice modes is improved significantly by including the linear terms.
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