Basis Set Extrapolation of Vibrational Frequencies

We investigate three different approaches for extrapolating harmonic vibrational frequencies to the complete basis set limit, by direct extrapolation of the frequencies, by calculation of frequencies based on extrapolated Hessians, or based on the Hessian from optimization of the extrapolated energy surface. For regular molecules, the three extrapolation procedures yield essentially identical results, but for loosely bound complexes, the frequencies derived from extrapolated Hessians yield unpredictable behavior. None of the basis set extrapolations, however, provide any significant improvement over the results upon which the extrapolation is based.本研究探讨了三种将简谐振动频率（harmonic vibrational frequencies）外推至完备基组极限（complete basis set limit）的方法：直接对频率进行外推、基于外推后的海森矩阵（Hessian）计算频率，以及基于外推能量面优化所得的海森矩阵进行计算。对于常规分子而言，三种外推流程的计算结果基本一致；但对于松散结合复合物（loosely bound complexes），由外推海森矩阵得到的振动频率则会表现出不可预测的行为。然而，所有基组外推方法均未比其依托的原始计算结果带来显著提升。