The very good performance of modern density functional theory for molecular geometries and harmonic vibrational frequencies has been well established. We investigate the performance of density functional theory (DFT) for quartic force fields, vibrational anharmonicity and rotation-vibration coupling constants, and thermodynamic functions beyond the RRHO (rigid rotor-harmonic oscillator) approximation of a number of small polyatomic molecules. Convergence in terms of basis set, integration grid and the numerical step size for determining the quartic force field by using central differences of analytical second derivatives has been investigated, as well as the performance of various exchange-correlation functionals. DFT is found to offer a cost-effective approach with manageable scalability for obtaining anharmonic molecular properties, and particularly as a source for anharmonic zero-point and thermal corrections for use in conjunction with benchmark {\it ab initio} thermochemistry methods.
Anharmonic force fields of perchloric acid, HClO$_4$, and perchloric anhydride, Cl$_2$O$_7$. An extreme case of inner polarization
用密度泛函理论（DFT）计算了高氯酸（HClO）$_4$和高氯酸酐（Cl $_2$ O $_7$）分子在Kohn-Sham极限附近的非谐力场.计算的基频与现有的实验数据非常吻合。根据计算结果，对Cl $_2$ O $_7$的振动光谱提出了一些解释. HClO $_4$和Cl $_2$ O $_7$是“内极化”现象的特别严重的例子。极化一致基组pc-1和pc-2（以及它们的增广对应物）应分别在第二行原子上补充两个（优选三个）和一个（优选两个）高指数$d$函数。完整的非谐力场可作为电子支持信息。
Methodologies for creating reactive potential energy surfaces from molecular mechanics force-fields are becoming increasingly popular. To date, molecular mechanics force-fields use harmonic expressions to treat bonding stretches, which is a poor approximation in reactive molecular dynamics simulations since bonds are displaced significantly from their equilibrium positions. For such applications there is need for a better treatment of anharmonicity. In this contribution Morse bonding potentials have been extensively parameterised for the atom types in the MM3 force field of Allinger and co-wor…
Assessment of Various Density Functionals and Basis Sets for the Calculation of Molecular Anharmonic Force Fields
In a previous contribution (Mol. Phys. { \bf 103}, xxxx, 2005), we established the suitability of density functional theory (DFT) for the calculation of molecular anharmonic force fields. In the present work, we have assessed a wide variety of basis sets and exchange-correlation functionals for harmonic and fundamental frequencies , equilibrium and ground-state rotational constants, and thermodynamic functions beyond the RRHO (rigid rotor-harmonic oscillator) approximation. The fairly good performance of double-zeta plus polarization basis sets for frequencies results from an error compensatio…
An anharmonic liquid-entropy functional from the Mori-Zwanzig memory kernel
University of California San Diego（加州大学圣地亚哥分校）；ATLAS Materials Physics Laboratory, Aiiso Yufeng Li Family Department of Nano and Chemical Engineering, University of California San Diego（加州大学圣地亚哥分校艾索·于峰李家族纳米与化学工程实验室）
Several methods are available to compute the anharmonicity in semi-rigid molecules. However, such methods are not routinely employed yet because of their large computational cost, especially for large molecules. The potential energy surface is required and generally approximated by a quartic force field potential based on ab initio calculation, thus limiting this approach to medium-sized molecules. We developed a new, fast and accurate hybrid Quantum Mechanic/Machine learning (QM//ML) approach to reduce the computational time for large systems. With this novel approach, we evaluated anharmonic…
We introduce vibrational heat-bath configuration interaction (VHCI) as an accurate and efficient method for calculating vibrational eigenstates of anharmonic systems. Inspired by its origin in electronic structure theory, VHCI is a selected CI approach that uses a simple criterion to identify important basis states with a pre-sorted list of anharmonic force constants. Screened second-order perturbation theory and simple extrapolation techniques provide significant improvements to variational energy estimates. We benchmark VHCI on four molecules with 12 to 48 degrees of freedom and use anharmon…
