Parallel computation of analytic second derivatives with applications to benzene and annulene
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CCSD(T) has been used in the past to accurately predict compute the spectra and structures of small molecules. However, the large execution times required for these calculations has limited their use in larger molecules such as benzene and annulene. The parallelization of analytic second derivatives of post Hartree-Fock methods, including CCSD(T), has enabled the VPT2+D treatment of the vibrational states of benzene. The fundamental frequencies and infrared active two quantum transitions that result are within 20 cm⁻¹ of the experimental values when treated for Fermi and Darling-Dennison resonances and empirical estimates for the harmonic frequencies and equilibrium bond lengths are determined to be within 12 cm⁻¹ and 0.004 Å of the values at the CCSD(T)/ANO2(fc) level of theory. The parallelization also facilitated the identification of two candidates for the structures of two isomers of annulene. The harmonic frequencies of several conformations proposed in the literature were computed at the CCSD/DZP level of theory with five of the conformations being ground states. The NMR shifts of four of these structures were computed using CCSD(T)/tzp and conformation 6(C₂ "twist") was identified as the likeliest structure for one of the isomers isolated. The remaining compound was identified as conformation 2b(C₂ "boat") due to a low conformation barrier and the proximity of its average NMR shifts to experiment. The identification of both compounds rely on properties computed using analytic derivatives not entirely on the relative energies of optimized geometries.