Ab Initio Born-Oppenheimer Molecular Dynamics Research Articles

A theoretical study of aqueous solvation of K comparing ab initio, polarizable, and fixed-charge models.

The hydration of K(+) is studied using a hierarchy of theoretical approaches, including ab initio Born-Oppenheimer molecular dynamics and Car-Parrinello molecular dynamics, a polarizable force field model based on classical Drude oscillators, and a nonpolarizable fixed-charge potential based on the TIP3P water model. While models based more directly on quantum mechanics offer the possibility to account for complex electronic effects, polarizable and fixed-charges force fields allow for simulations of large systems and the calculation of thermodynamic observables with relatively modest computational costs. A particular emphasis is placed on investigating the sensitivity of the polarizable model to reproduce key aspects of aqueous K(+), such as the coordination structure, the bulk hydration free energy, and the self diffusion of K(+). It is generally found that, while the simple functional form of the polarizable Drude model imposes some restrictions on the range of properties that can simultaneously be fitted, the resulting hydration structure for aqueous K(+) agrees well with experiment and with more sophisticated computational models. A counterintuitive result, seen in Car-Parrinello molecular dynamics and in simulations with the Drude polarizable force field, is that the average induced molecular dipole of the water molecules within the first hydration shell around K(+) is slightly smaller than the corresponding value in the bulk. In final analysis, the perspective of K(+) hydration emerging from the various computational models is broadly consistent with experimental data, though at a finer level there remain a number of issues that should be resolved to further our ability in modeling ion hydration accurately.

Electron-corrected Lorentz forces in solids and molecules in a magnetic field

We describe the effective Lorentz forces on the ions of a generic insulating system in an magnetic field, in the context of Born-Oppenheimer ab-initio molecular dynamics. The force on each ion includes an important contribution of electronic origin, which depends explicitly on the velocity of all other ions. It is formulated in terms of a Berry curvature, in a form directly suitable for future first principles classical dynamics simulations based {\it e.g.,} on density functional methods. As a preliminary analytical demonstration we present the dynamics of an H$_2$ molecule in a field of intermediate strength, approximately describing the electrons through Slater's variational wavefunction.

Ab initio molecular dynamics evidence of a new stable symmetric C s structure for N(OH) 3

We have recently shown that the C 1 structure of N(OH) 3 to be more stable than the C 3 by only around 1 kcal/mol. We study here the dynamical process each of these structures follows through Born–Oppenheimer ab initio molecular dynamics simulations at room temperature, using gaussian basis sets and a hybrid exchange-correlation functional; the structural evolution of both processes lead to the discovery of a new and more stable isomer with C s symmetry. Highly-correlated calculations at the MP2, MP4 and CCSD(T) levels confirm this new structure as the absolute minimum in the global PES of N(OH) 3.

Constrained dynamics and extraction of normal modes from ab initio molecular dynamics: Application to ammonia

We present a methodology for extracting phonon data from ab initio Born-Oppenheimer molecular dynamics calculations of molecular crystals. Conventional ab initio phonon methods based on perturbations are difficult to apply to lattice modes because the perturbation energy is dominated by intramolecular modes. We use constrained molecular dynamics to eliminate the effect of bond bends and stretches and then show how trajectories can be used to isolate and define in particular, the eigenvalues and eigenvectors of modes irrespective of their symmetry or wave vector. This is done by k-point and frequency filtering and projection onto plane wave states. The method is applied to crystalline ammonia: the constrained molecular dynamics allows a significant speed-up without affecting structural or vibrational modes. All Gamma point lattice modes are isolated: the frequencies are in agreement with previous studies; however, the mode assignments are different.

Fock matrix dynamics

An efficient method is suggested for direct ab initio Born–Oppenheimer molecular dynamics. It is based on extrapolating the Fock matrices from previous time steps to provide a start for the SCF procedure. Several polynomial extrapolation schemes are compared; low-degree schemes work best. SCF convergence can be achieved in 2–3, sometimes less than two cycles. We have identified a phenomenon, hysteresis, a violation of the energy conservation and reversibility, due to systematic errors arising from incomplete SCF convergence. It can be eliminated by rescaling the velocities. Examples include water clusters and hydrated halogen ions, running simulations for several picoseconds.