Abstract：
Vibrational spectroscopy enables critical insight into the structural and dynamic properties of molecules. Presently, the majority of theoretical approaches to spectroscopy employ wavefunction-based ab initio or density functional methods that rely on the harmonic approximation. This approximation breaks down for large molecules with strongly anharmonic bonds or for molecules with large internuclear separations. An alternative to these methods involves generating molecular anharmonic potential energy surfaces (potentials) and using them to extrapolate the vibrational frequencies. This study examines the efficacy of density functional theory (DFT) and the correlation consistent Composite Approach (ccCA) in generating anharmonic frequencies from potentials of small main group molecules. Vibrational self-consistent field Theory (VSCF) and post-VSCF methods were used to calculate the fundamental frequencies of these molecules from their potentials. Functional choice, basis set selection, and mode-coupling are also examined as factors in influencing accuracy. The absolute deviations for the calculated frequencies using potentials at the ccCA level of theory were lower than the potentials at the DFT level. With DFT resulting in bending modes that are better described than those of ccCA, a multilevel DFT:ccCA approach where DFT potentials are used for single vibrational mode potentials and ccCA is used for vibrational mode-mode couplings can be utilized for larger polyatomic systems. The frequencies obtained with this multilevel approach using VCIPSI-PT2 were closer to experimental frequencies than the scaled harmonic frequencies, indicating the success of utilizing post-VSCF methods to generate more accurate representations of computed infrared spectra.
摘要：
振动光谱学能够深入了解分子的结构和动态特性。目前,光谱学的大多数理论方法都采用基于波函数的从头算或依赖于谐波近似的密度泛函方法。对于具有强烈非谐键的大分子或具有大的核间分离的分子，这种近似分解。这些方法的替代方法包括生成分子非谐势能面（电势）并使用它们来推断振动频率。这项研究考察了密度泛函理论（DFT）和相关一致复合方法（ccCA）在从小主族分子的电势产生非谐波频率方面的功效。振动自洽场论（VSCF）和后VSCF方法用于根据这些分子的电势计算它们的基本频率。功能选择,基集选择，和模式耦合也被视为影响精度的因素。使用理论的ccCA水平的电势计算的频率的绝对偏差低于DFT水平的电势。DFT导致的弯曲模式比ccCA更好地描述，多水平DFT:ccCA方法,其中DFT电势用于单振动模式电势,并且ccCA用于振动模式-模式耦合,可以用于更大的多原子系统。使用VCIPSI-PT2的这种多级方法获得的频率比缩放谐波频率更接近实验频率，表明成功利用后VSCF方法来生成更准确的计算红外光谱表示。
