Gupta V P
Department of Physics, University of Jammu, Jammu-Tawi 180006, India.
Spectrochim Acta A Mol Biomol Spectrosc. 2007 Jul;67(3-4):870-6. doi: 10.1016/j.saa.2006.09.002. Epub 2006 Sep 10.
The paper reports main results of a comprehensive study of the vibrational spectrum of ketene computed using second-order perturbation theory treatment based on quartic, cubic and semidiagonal quartic force constants. Two different models--a homogeneous model using the same density functionals and basis functions for the harmonic calculations and anharmonic corrections, and a hybrid model in which the two parts of the calculation are conducted using different density functionals and basis sets--have been employed in the present calculations. Different DFT and CCSD methods and DZ and TZ extended basis sets involving diffuse and polarization functions have been used to calculate optimized and vibrationally averaged geometrical parameters, the harmonic and anharmonic vibrational frequencies and the spectroscopic constants such as anharmonicity constants, rotational constants, rotation-vibration coupling constants, Nielsen's centrifugal distortion constants and Coriolis coupling constants. Homogeneous model is found to be superior to the hybrid model in several respects. Difficulties in the hybrid model may arise due to one of the following reasons: (a) the basic requirement that the geometry optimization and frequency calculations must be done at the same level of theory to have valid frequencies is not met in the hybrid model; (b) the molecular structure gets reoptimized at the low level for anharmonic corrections; (c) in addition, the perturbation could also diverge for the above reasons, particularly for the very low, very anharmonic terms where the harmonic approximation is not close enough to make the perturbation work.
本文报道了基于四次、三次和半对角四次力常数,采用二阶微扰理论处理方法计算乙烯酮振动光谱的综合研究的主要结果。本计算采用了两种不同的模型:一种是均匀模型,在谐波计算和非谐校正中使用相同的密度泛函和基函数;另一种是混合模型,其中计算的两个部分使用不同的密度泛函和基组。使用了不同的密度泛函理论(DFT)和耦合簇单双激发(CCSD)方法以及包含弥散和极化函数的双ζ(DZ)和三ζ(TZ)扩展基组,来计算优化的和振动平均的几何参数、谐波和非谐振动频率以及光谱常数,如非谐性常数、转动常数、转动 - 振动耦合常数、尼尔森离心畸变常数和科里奥利耦合常数。发现均匀模型在几个方面优于混合模型。混合模型中可能由于以下原因之一而出现困难:(a)混合模型未满足几何优化和频率计算必须在相同理论水平上进行以获得有效频率的基本要求;(b)分子结构在低水平上重新优化以进行非谐校正;(c)此外,由于上述原因,微扰也可能发散,特别是对于非常低、非常非谐的项,其中谐波近似不够接近以使微扰起作用。