title: Perturbative treatment of anharmonic vibrational effects on bond distances: an extended Langevin dynamics method.
abstract: In this work, we first review the perturbative treatment of an oscillator with cubic anharmonicity. It is shown that there is a quantum-classical correspondence in terms of mean displacement, mean-squared displacement, and the corresponding variance in the first-order perturbation theory, provided that the amplitude of the classical oscillator is fixed at the zeroth-order energy of quantum mechanics EQM (0). This correspondence condition is realized by proposing the extended Langevin dynamics (XLD), where the key is to construct a proper driving force. It is assumed that the driving force adopts a simple harmonic form with its amplitude chosen according to EQM (0), while the driving frequency chosen as the harmonic frequency. The latter can be improved by using the natural frequency of the system in response to the potential if its anharmonicity is strong. By comparing to the accurate numeric results from discrete variable representation calculations for a set of diatomic species, it is shown that the present method is able to capture the large part of anharmonicity, being competitive with the wave function-based vibrational second-order perturbation theory, for the whole frequency range from ∼4400 cm(-1) (H2 ) to ∼160 cm(-1) (Na2 ). XLD shows a substantial improvement over the classical molecular dynamics which ceases to work for hard mode when zero-point energy effects are significant.
authors.affiliation: Department of chemistry, State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, People's Republic of China.
authors.forename: Tonghao
authors.initials: T
authors.lastname: Shen
authors.forename: Neil Qiang
authors.initials: NQ
authors.lastname: Su
authors.forename: Anan
authors.initials: A
authors.lastname: Wu
authors.forename: Xin
authors.initials: X
authors.lastname: Xu
keywords: Langevin dynamics; anharmonic effect; molecular dynamics; molecular structure; zero-point energy