Ab initio molecular dynamics: Theory and Implementation

level by various chain and ring structures that can be statistically analyzed in abinitio molecular dynamics simulations 631 . Liquid GeSe 2 is characterized by strongchemical bonds that impose a structure beyond the usual very short distances dueto network formation 416 . Zintl–alloys such as liquid NaSn 552 or KPb 556 have veryinteresting bonding properties that manifest themselves in strong temperature– andconcentration dependences of their structure factors (including the appearance ofthe so–called first sharp diffraction peak 555 ) or electric conductivities.Metals are ideal systems to investigate the metal–insulator transition upon expansionof the liquid 346,63 or melting 689 . Liquid copper was simulated at 1500 K:structural and dynamical data were found to be in excellent agreement with experimental464 . Transport coefficients of liquid metals (including in particular extremeconditions) can also be obtained from first principles molecular dynamics using theGreen–Kubo formalism 571,592 . The microscopic mechanism of the semiconductor–metal transition in liquid As 2 Se 3 could be rationalized in terms of a structuralchange as found in ab initio simulations performed as a function of temperatureand pressure 563 . The iii–v semiconductors, such as GaAs, assume metallic behaviorwhen melted, whereas the ii–vi semiconductor CdTe does not. The differentconductivities could be traced back to pronounced structural dissimilarities of thetwo systems in the melt 236 .5.5 Glasses and Amorphous SystemsRelated to the simulation of dynamically disordered fluid systems are investigationsof amorphous or glassy materials. In view of the severe limitations on systemsize and time scale (and thus on correlation lengths and times) ab initio moleculardynamics can only provide fairly local information in this sense. Within these inherentconstraints the microscopic structure of amorphous selenium 304 and tetrahedralamorphous carbon 384 , the amorphization of silica 684 , boron doping in amorphousSi:H 181 or in tetrahedral amorphous carbon 227 , as well as the Raman spectrum 465and dynamic structure factor 466 of quartz glass and their relation to short–rangeorder could be studied.The properties of supercooled CdTe were compared to the behavior in the liquidstate in terms of its local structure 237 . Defects in amorphous Si 1−x Ge x alloysgenerated by ab initio annealing were found to explain ESR spectra of this system329 . The infrared spectrum of a sample of amorphous silicon was obtainedand found to be in quantitative agreement with experimental data 152 . The CO 2insertion into a model of argon–bombarded porous SiO 2 was studied 508 . In particularthe electronic properties of amorphous GaN were investigated using ab initiomethods 601 .Larger systems and longer annealing times are accessible after introducing moreapproximations into the first principle treatment of the electronic structure thatunderlies ab initio molecular dynamics. Using such methods 551 , a host of differentamorphous carbon nitride samples with various stoichiometries and densities couldbe generated and characterized in terms of trends 675 . Similarly, the pressure–induced glass–to–crystal transition in condensed sodium was investigated 22 and twostructural models of amorphous GaN obtained at different densities were examined122