Ab initio molecular dynamics: Theory and Implementation

the penetration of the oxidation layer into the bulk. Similarly, the growth of anoxide layer was generated on an Si(100) surface 653 .The water–Pd(100), water–O/Pd(100) and water–Si(111) interfaces were simulatedbased on ab initio molecular dynamics 336,655 . Water covering the surface ofa microscopic model of muscovite mica is found to form a two–dimensional networkof hydrogen bonds, called two–dimensional ice, on that surface 447 . The metal–organic junction of monolayers of Pd–porphyrin and perylene on Au(111) was analyzedusing an ab initio approach 355 . An interesting possibility is to compute thetip–surface interactions in atomic force microscopy as e.g. done for a neutral silicontip interacting with an InP(110) surface 619 or Si(111) 481,482 .5.4 Liquids and SolutionsMolecular liquids certainly belong to the classic realm of molecular dynamics simulations.Water was and still is a challenge 581 for both experiment and simulationsdue to the directional nature and the weakness of the hydrogen bonds which leadsto delicate association phenomena. Pioneering ab initio simulations of pure waterat ambient 352 and supercritical conditions 205 were reported only a few yearsago. More recently, these gradient–corrected density functional theory–based simulationswere extended into several directions 587,573,575,576,579,118 . In the mean time(minimal–basis) Hartree–Fock ab initio molecular dynamics 291 as well as more approximateschemes 455 were also applied to liquid water. Since chemical reactionsoften occur in aqueous media the solvation properties of water are of utmost importanceso that the hydration of ions 403,620,621,377,502 and small molecules 353,354,433was investigated. Similarly to water liquid HF is a strongly associated liquid whichfeatures short–lived hydrogen–bonded zig–zag chains 521 . Another associated liquid,methanol, was simulated at 300 K using an adaptive finite–element method 634in conjunction with Born–Oppenheimer molecular dynamics 635 . In agreement withexperimental evidence, the majority of the molecules is found to be engaged in shortlinear hydrogen–bonded chains with some branching points 635 . Partial reviews onthe subject of ab initio simulations as applied to hydrogen–bonded liquids can befound in the literature 586,406,247 .The ab initio simulated solvation behavior of “unbound electrons” in liquidammonia at 260 K was found to be consistent with the physical picture extractedfrom experiment 155,156 . Similarly,ab initio molecular dynamics of dilute 553,203 andconcentrated 569 molten K x·(KCl) 1−x mixtures were performed at 1300 K enteringthe metallic regime. The structure of liquid ammonia at 273 K was investigatedwith a variety of techniques so that limitations of using classical nuclei, simple pointcharge models, small systems, and various density functionals could be assessed 164 .Ab initio molecular dynamics is also an ideal tool to study other complex fluidswith partial covalency, metallic fluids, and their transformations as a function oftemperature, pressure, or concentration. The properties of water–free KF • nHFmelts depend crucially on polyfluoride anions H m F − m+1 and solvated K+ cations.Ab initio simulations allow for a direct comparison of these complexes in the liquid,gaseous and crystalline phase 515 . The changes of the measured structure factorof liquid sulfur as a function of temperature can be rationalized on the atomistic121