Basis vector equilibrium calculations

MBPT(2) has also been applied to calculate vibrational frequencies of polymers. With the translational symmetry, one can only calculate the vibrational modes with the reciprocal vector k = 0. These modes are of particular importance since they give rise to infrared and Raman spectra [67]. We applied MBPT(2) to polymethineimine and calculated its equilibrium structure, band gap, and vibrational frequencies with basis sets STO-3G, 6-31G and 6-31G [68]. Both basis set and electron correlation have a strong influence on its vibrational frequencies as well as its optimized geometry and band gap. With respect to in-phase (k=0) nuclear displacements, Hirata and Iwata very recently calculated the MBPT(2) vibrational frequencies of polyacetylene for basis sets STO-3G and 6-31G with analytical gradients [69], They showed that MBPT(2) greatly improves the HF vibrational frequencies for polyacetylene. 

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