Ab initio dynamics

Truong T N 1997 Thermal rates of hydrogen exchange of methane with zeolite a direct ab initio dynamics study on the importance of quantum tunneling effects J. Rhys. Chem. B 101 2750... 

Radeke M R and Carter E A 1997 Ab initio dynamics of surface chemistry Ann. Rev. Phys. Chem. 48 243-70... 

In principle, we could find the minimum-energy crystal lattice from electronic structure calculations, determine the appropriate A-body interaction potential in the presence of lattice defects, and use molecular dynamics methods to calculate ab initio dynamic macroscale material properties. Some of the problems associated with this approach are considered by Wallace [1]. Because of these problems it is useful to establish a bridge between the micro-... 

Truong, T. N. and W. Dunkan. 1994. A new direct ab initio dynamics method for calculating thermal rate constants from density-functional theory. J. Chem. Phys. 101, 7408. 

Bell, R.L. and T.N. Truong. 1994. Direct ab initio dynamics studies of proton transfer in hydrogen-bond systems. J. Chem. Phys. 101, 10442. 

Dependent Rate Constants of Multichannel Unimolecular Decomposition of Gas-phase a-HMX An Ab Initio Dynamics Study. 

Figure 1. Intramolecular vibrational density redistribution IVR of Na3

In fact, the distinction between two-step and direct dynamics is rather fuzzy. The basic issue is what kind and amount of preliminary work is needed before starting a dynamical calculation. Direct ab initio dynamics [90,97-101] requires a minimum of preparation some tests to choose basis sets and other options may suffice. For large systems, however, fully ab initio calculations are impractical, and one has to resort to QM/MM or PCM approaches but then, a host of empirical parameters are introduced, which may need some readjustement to avoid artefacts and to improve the accuracy before starting the dynamical calculations. The same holds for the semiempirical methods in order to represent at best the excited states, one has to re-parameterize the hamiltonian. In particular, our FOMO-SCF-CI method [56-58] differs considerably from the normal SCF or SCF+CIS procedures, so that the standard parameters need to be modified. However, the parameter sets are fairly transferable, and their optimization can be limited to the atoms belonging to the chromophore. In the two-step strategies one fits the ab... 

H. Tachikawa, Ionization dynamics of the benzene-HF complex A direct ab initio dynamics study. J. Phys. Chem. A 103, 6873-6879 (1999)... 

Before a detailed presentation of the ab initio dynamics simulations, first the fundamental difference between atomic and molecular adsorption on the one hand and dissociative adsorption on the other hand has to be addressed. Then I will briefly discuss the question whether quantum or classical methods are appropriate for the simulation of the adsorption dynamics. This section will be followed by a short introduction into the determination of potential energy surfaces from first principles and their continuous representation by some analytical or numerical interpolation schemes. Then the dissociative adsorption and associative desorption of hydrogen at metal and semiconductor surfaces and the molecular trapping of oxygen on platinum will be discussed in some detail. 

For any dynamical simulation, a continuous representation of the PES is mandatory since the potential and the gradients are needed for arbitrary configurations. One can in fact perform ab initio molecular dynamics simulations in which the forces necessary to integrate the classical equations of motion are determined in each step by an electronic structure calculations. There have been few examples for such an approach [35-37], However, in spite of the fact that electronic structure calculations can nowadays be performed very efficiently, still there is a significant numerical effort associated with ab initio calculations. This effort is so large that in the ab initio dynamics simulations addressing molecular adsorption and desorption at surfaces the number of calculated trajectories has been well below 100, a number that is much too low to extract any reliable reaction probabilities. 

Dr. McLaughl, D.L. Thompson, Ab-initio dynamics HeH+ -h H2 He -h (C2v) classical trajectories using a quantum-mechanical potential-energy surface, /. Chem. Phys. 59 (8) (1973) 4393-4405. 

Direct Ab Initio Dynamics Methodology for Modeling Kinetics of Biological Systems (T. N. Truong D. K. Maity)... 

Spelsberg, D. and Meyer, W. ( 992i)Ab initio dynamic dipole polarizabilities for O2, J. Chem. Phys. 109, 9802-9810. 

Reaction path methods have great promise for future progress because they can be made direct or automated. Direct ab initio dynamics is taken to mean the ab initio evaluation of the molecular energy whenever it is required in a dynamical calculation. At present this is prohibitively expensive. A more traditional approach uses electronic structure methods to provide information about a PES which is then fitted to a functional form. The dynamics employed thereafter is restricted only by the limitations of current dynamical theories. However, the process of fitting functional forms to a molecular PES is difficult, unsystematic, and extremely time consuming. The reaction path approaches that use interpolation of the reaction path data are affordable methods aimed toward direct dynamics, as they avoid the process of fitting a functional form for the PES. Such methods have been automated (programmed) and are therefore readily usable. Thus reaction path methods have been applied with various levels of ab initio theory to statistical theories of the reaction rate, to approximate quantum dynamics, and to classical trajectory studies of reactions. 

Theoretical chemistiy is currently in a stage that experts in the field characterize as " the primitive beginnings of chemical ab initio dynamic The majority of the systems studied so far are three-atomic ... 

Rate Constants and the Kinetie Isotope Effeets in Multi-Proton Transfer Reactions A Case Study of CIONO2 + HCl HNO3 + Cl2 Reactions with Water Clusters with Canonical Variational Transition State Theory using a Direct Ab Initio Dynamics Approach... 

Our dynamical spectra thus clearly suggest that the two isomers 2A and 2B of CP-(Methanol)2 are simultaneously present in the experiment. Our dynamics and the RRKM calculations presented in [65] show that they do not spontaneously interconvert over the short 10 ps timescale of the ab initio dynamics, but that they can possibly interconvert over the hundred picosecond and nanosecond timescales at the 100 K temperature relevant to the experiment. 

Gross A (1998) Reactions at surfaces studied by ab initio dynamics calculations. Surf Sd Rep 32 291... 

Coupling of Length Scales Method Atomistic/Continuum Part The FE/MD/ TB coupling of length scales (CLS) method, " " " "° "" also known as MAAD (macroscopic, atomistic, ab initio dynamics), is one of the few methodologies to provide a dynamical coupling of three different regimes the macroscopic... 

Several basis sets were examined in order to select the most convenient and best fitted set for high-quality calculations. The Hartree-Fock (HF) 3-21 -I- G(d) calculation reasonably represents a potential-energy surface calculated at the MP2/6-311-I--I- G(2df,2pd) level. A full-dimensional ab initio potential-energy surface including all degrees of freedom was used in a ab initio dynamics calculation at the HF/3-21 - -G(d) level. Such dynamics calculations were further pursued in a later paper. A functional representation of the potential-energy surface for the reaction has been discussed and the study extended to the reaction... 

Expression 11.3 provides the whole simulated spectrum, while a detailed vibrational analysis requires the unambiguous assignment of each mode contribution. Recently, a number of methods appeared in the literature aimed at the extraction of normal-mode-like analysis from ab initio dynamics [58-63]. Some of these [58-60] refer to the quasi-harmonic model introduced by Karplus [64,65] in the framework of classical molecular dynamics and individuate normal-mode directions as main components of the nuclear fluctuations in the NVE or NVT ensemble. The quasinormal model relies on the equipartition of the kinetic energy among normal modes thus problems arise when the simulation time required to obtain such a distribution is computationally too expensive, as is often the case for ab initio dynamics. Other approaches [61-63] carry out the time evolution analysis in the momenta subspace instead of the configurational space. In these approaches the basic consideration is that, at any temperature, generalized normal modes g, correspond to uncorrelated momenta such that [61]... 

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