ABINITIO CALCULATIONS OF POLARIZABILITIES AND 2ND HYPERPOLARIZABILITIES OF ORGANIC-MOLECULES WITH EXTENDED PI-ELECTRON CONJUGATION
ABINITIO CALCULATIONS OF POLARIZABILITIES AND 2ND HYPERPOLARIZABILITIES OF ORGANIC-MOLECULES WITH EXTENDED PI-ELECTRON CONJUGATION
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DOI：
10.1021/j100357a020
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发表时间：
1989-10-05
影响因子：
--
通讯作者：
PRASAD, PN
中科院分区：
文献类型：
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作者：
CHOPRA, P;CARLACCI, L;PRASAD, PN
用从头算SCF理论计算了一系列聚烯、聚烯和积云的静态极化张量和二次超极化张量。数值稳定的有限场（FF）计算可以通过使用能量或感应偶极矩作为场强函数的多项式来实现。这些拟合的非线性膨胀系数对应于微观非线性光学性质。完全耦合（FF）从头计算极化率的结果与非耦合（态和）从头计算方法的结果很好地一致。超极化也不能比较。用经验函数的形式定性地描述了中长链的极化率和超极化率与链长之间的关系。漫射轨道基函数对于小共轭体系或任何小分子的定性正确的超极化率是必需的。例如，在STO-3G、6-31G和增强的3-21G基组下，乙烯的平均秒超极化率7分别为13、1.7和726 au。用增广基集计算的值与1500au的实验值在2倍的范围内一致。碳原子的价电子集被扩充为弥散s， p和两个笛卡尔d集，或它们的子集。漫射偏振函数的加入极大地改变了计算的二阶超极化率。结果对价集的选择几乎不敏感，但高度依赖于基集的质量。我们还描述了使用相应的轨道分析来帮助解释由FF或解析导数方法获得的从头计算结果。计算得到的极化率和超极化率以及和分量表明，轨道的贡献要比轨道的贡献大得多。从乙烯链和乙炔链到积云体系，两者的贡献都发生了变化。通过导数电子密度函数的等高线面来说明电场对电子密度的极化作用。对于乙炔链，电荷密度的一阶导数相对于场(a)的等高线图几乎是周期性的，对应于局域极化，而三阶导数密度(7)对应于更远距离的电荷位移。
Static polarizability and second hyperpolarizability tensors are computed for a series of polyenes, polyynes, and cumulenes by ab initio SCF theory. Numerically stable finite field (FF) calculations can be achieved by using polynomialfits of either energy or induced dipole moment as a function of field strength. The nonlinear expansion coefficients from these fits correspond to the microscopic nonlinear optical property. Our results from fully coupled (FF) ab initio calculations for polarizability are in good agreement with those derived from uncoupled (sum-over-states) ab initio methods. The hyperpolarizabilities do not compare as well. A qualitative description of the chain length dependence of polarizability and hyperpolarizability for moderately long chains is discussed in terms of an empirical function. Diffuse orbital basis functions are required for qualitatively correct hyperpolarizabilities of small conjugated systems or for that matter any small molecule. For example, the average second hyperpolarizability, 7, of ethylene is computed to be-13, 1.7, and 726 au with STO-3G, 6-31G, and augmented 3-21G basissets, respectively. The value computed with the augmented basisset agrees, within a factor of 2, with the experimental value of 1500 au. The valence set for the carbon atoms is augmented with diffuse s, p, and two Cartesian d sets, or subsets of these. The inclusion of diffuse polarization functions drastically alters the computed second hyperpo-larizabilities. Theresults are nearly insensitive to the choice of valence set but are highly dependent on the basis set quality. We also describe the use of a corresponding orbital analysis to aid in the interpretationof ab initio results obtained by either FF or analytic derivativemethods. The computed polarizability and hyperpolarizability and the and components indicate that the contribution due to the orbitals is much more significant than the orbitals. Both contributions change sign in going from the ethylenic and acetylenic chains to the cumulenic systems. The polarizationof-electron density by the field is illustrated by contour surfaces of derivative-electron-density functions. Contour maps of the first derivative of charge density with respect to thefield (a) for an acetylenic chain are nearly periodic, corresponding to localized polarization, whereas the third derivative density (7) corresponds to longer range charge shifts.