# Ab Initio Path Integral Molecular Dynamics

#### Quantum and Thermal Fluctuation Effects on the Photoabsorption Spectra of Clusters

Physical Review Letters, 92(18):183401.
Fabio D Sala, Roger Rousseau, Andreas Görling, and Dominik Marx (2004)

Abstract

A first principles approach for the calculation of photoabsorption cross sections of clusters and molecules is presented which includes both quantum and thermal fluctuations. The method relies on an ab initio path integral representation of the nuclear quantum motion in conjunction with time-dependent density-functional theory for electronic excitations. It is shown that quantum fluctuations of cold lithium clusters, such as Li8 below 50 K, affect significantly their photoabsorption spectra.

#### Quantum dynamics via adiabatic ab initio centroid molecular dynamics

Computer Physics Communications, 118(2-3):166–184.
Dominik Marx, Mark E Tuckerman, and Glenn J Martyna (1999)

Abstract

The ab initio path integral simulation method is combined with centroid molecular dynamics. This unification, and thus extension of these basic techniques, allows for the investigation of the real-time quantum dynamics in chemically complex many-body systems. The theory underlying the proposed ab initio centroid molecular dynamics (AICMD) technique is presented in detail. The real-time propagation of the nuclei is obtained in the quasiclassical approximation within the framework of centroid path integrals. Concurrently, the forces acting on the nuclei are computed from “on the fly” electronic structure calculations based on first-principle techniques such as, e.g., Hohenberg—Kohn—Sham density functional theory. AICMD can be considered as a quasiclassical generalization of standard Car—Parrinello ab initio molecular dynamics. At the same time, AICMD preserves the virtues of the ab initio path integral technique to generate exact time-independent quantum equilibrium averages. AICMD is well suited to investigate, in a quasiclassical sense, the real-time evolution of molecular quantum systems with complex interactions which cannot be satisfactorily represented by simple model potentials. In particular, the method permits the simulation of the dynamics of chemical reactions including quantum effects. AICMD is applicable to isolated systems in the gas phase such as molecules, clusters or complexes as well as to condensed matter, i.e. molecular liquids or solids. Keywords: Real-time quantum dynamics; Ab initio path integral simulations; Centroid molecular dynamics; Quasiclassical time evolution; Chemical reactions; First principles simulations PACS: 31.15.Kb; 67.20.+k; 03.65.Sq; 31.15.Qg; 05.60.Gg; 31.15.Gy

URL: http://dx.doi.org/10.1016/S0010-4655(99)00208-8

#### Ab initio path integral molecular dynamics: Basic ideas

The Journal of Chemical Physics, 104(11):4077-4082.
Dominik Marx and Michele Parrinello (1996)

Abstract

The essential ideas underlying ab initio path integral molecular dynamics and its efficient numerical implementation are discussed. In this approach the nuclei are treated as quantum particles within the path integral formulation of quantum statistical mechanics. The electronic degrees of freedom are treated explicitly based on state‐of‐the‐art electronic structure theory. This renders ab initio simulations of quantum systems possible without recourse to model potentials. A combined extended Lagrangian for both quantum nuclei and electrons defines a dynamical system and yields molecular dynamics trajectories that can be analyzed to obtain quantum statistical expectation values of time‐independent operators. The methodology can be applied to a wide range of fields addressing problems in molecular and condensed matter chemistry and physics.

URL: http://dx.doi.org/10.1063/1.471221