First-principles quantum chemical

It is thus obvious that among numerous computational methods, first principles quantum chemical approach is indispensable. However, initially first principles quantum chemical calculations required the use of models consisting of a few atoms (clusters) and the range of properties was limited. Since the advent of modem computing resources, as well the models could be extended to cover larger variety of structures as the methodology has been... 

During the last decade, density-functional theory (DFT)-based approaches [1, 2] have advanced to prominent first-principles quantum chemical methods. As computationally affordable tools apt to treat fairly extended systems at the correlated level, they are also of special interest for applications in medicinal chemistry (as demonstrated in the chapters by Rovira, Raber et al. and Cavalli et al. in this book). Several excellent text books [3-5] and reviews [6] are available as introduction to the basic theory and to the various flavors of its practical realization (in terms of different approximations for the exchange-correlation functional). The actual performance of these different approximations for diverse chemical [7] and biological systems [8] has been evaluated in a number of contributions. 

Further theoretical studies supported by in situ spectroscopy and high-resolution microscopy are needed to be able to understand this unusually strong bonding between Cu and Ce. To apply such first-principles quantum chemical MD approach, new computational methods accelerating computational time by several orders of magnitude must be developed. 

Figure 2. Illustration of simulation techniques available at various time and length scales. QC means first principles, quantum chemical methods. MD refers to classical molecular dynamics methods. (Monte Carlo methods are useful in roughly the same range of time and distance.) Methods for connecting QC, MD, and continuum methods are indicated in parentheses.

In this chapter, I have provided a brief overview of the QMC method for electronic structure with emphasis on the more accurate diffusion Monte Carlo (DMC) variant of the method. The high accuracy of the approach for the computation of energies is emphasized, as well as the adaptability to large multiprocessor computers. Recent developments are presented that shed light on the capability of the method for the computation of systems larger than those accessible by other first principles quantum chemical methods. 

The final section provides an overview of the current understanding of the factors that govern the physical chemistry of chemisorption. Our understanding of the factors that determine the site preference of surface dependence of chemisorption is summarized. We demonstrate many of those concepts through a scries of first-principle quantum chemical results on different example systems. The results allow us to specifically quantify different aspects of the interaction, such as... 

First-principles quantum chemical calculations including relativistic effects have been carried out for dipole moments, polarizabilities, and first- and second-order hyperpolarizabilities for tellurophene. The estimated values were compared with the observed ones measured by the optical Kerr effects <2000SM185, 2003JMT207>. 

Cavalli, A., Carloni, P. and Recanatini, M. (2007) Target-related applications of first principles quantum chemical methods in drug design. Chem. Rev., 106, 3497-3519. 

Undoubtedly, the methods most widely used to solve the Schrodinger equation are those based on the approach originally proposed by Hartree [1] and Fock [2]. Hartree-Fock (HF) theory is the simplest of the ab initio or "first principles" quantum chemical theories, which are obtained directly from the Schrodinger equation without incorporating any empirical considerations. In the HF approximation, the n-electron wavefunction is built from a set of n independent one-electron spin orbitals which contain both spatial and spin components. The HF trial wavefunction is taken as a single Slater determinant of spin orbitals. 

Ghosh, A. (1998). First-principles quantum chemical studies of porphyrins. Acc. Chem. Res. 31, 189-198. 

First-principle quantum chemical methods have advanced to the stage where they can now offer qualitative, as well as, quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity for a series of relevant commercial chemistries. DFT-predicted adsorption and overall reaction energies were found to be within 5 kcal/mol of the experimentally known values for all systems studied. Activation barriers were over-predicted but still within 10 kcal/mol. More specifically we examined the mechanisms and reaction pathways for hydrocarbon C-H bond activation, vinyl acetate synthesis, and ammonia oxidation. Extrinsic phenomena such as substituent effects, bimetallic promotion, and transient surface precursors, are found to alter adsorbate-surface bonding and surface reactivity. 

In this paper, I review the recent advances and developments of first-principle quantum chemical methods and discuss their application to modelling chemisorption, surface reactivity of reactants/intermediates, and the catalytic behavior for a series of relevant commercial chemistries. We focus primarily on the static representation of the surface. 

First principle quantum chemical methods have reached the stage where they can now begin to provide reliable information on the structure, spectral and energetic properties of adsorbates on surfaces. Both the cluster, as well as the periodic band methods, were found to be quite successful. In contrast to early MO-based cluster studies, the DFT cluster methodology provides more accurate predictions, provided that special care is taken to optimize the structural configurations, the geometries, and spin states for all clusters. 

This obvious need for clarifying the relationship between the electronic and geometric stmcture of paramagnetic systems and their g values nowadays can be met with the help of first-principles quantum chemical methods. A theoretical description of electronic g values, based on a relativistic method which accurately treats spin-orbit interaction even in cases when it is too strong to be considered as a perturbation, will be uniformly applicable to systems with both light and heavy atoms. 

The purpose of this chapter is to selectively summarize recent advances in the molecular modeling of anode and cathode electrocatalytic reactions employing different computational approaches, ranging from first-principles quantum-chemical calculations (based on density functional theory, DFT), ab initio and classical molecular dynamics simulations to kinetic Monte Carlo simulations. Each of these techniques is associated with a proper system size and timescale that can be adequately treated and will therefore focus on different aspects of the reactive system under consideration. 

Advances in computational chemistry and molecular simulation have also reached the stage whereby they can be used to develop more advanced and robust kinetic models for catalytic systems. First-principle quantum chemical methods, for example, are being used to routinely calculate thermochemistry and kinetics for gas phase chemistry with accuracies on the order of... 

The detailed information offered from the simulation enables us to point out the molecular features that govern the chemistry. By calculating all of the barriers and reaction energetics from first-principle quantum chemical calculations, the simulations are free of any experimentally regressed parameters. The results for the simulation agree remarkably well with the known experimental data ( 50,31,52,53) simple power law rate model derived Irom the simulations is compared with that known from experiments below in Eqs. 4 and 5. 

First-principles quantum chemical methods have allowed elucidation of reaction mechanisms for a variety of heterogeneous catalytic reactions. As discussed above, incorporating the nature of the electrochemical double layer into quantum models is limited by the challenges associated with following the structure and dynamics of the electrolyte over the electrode. Further, to capture electro-catalytic reaction mechanisms accurately using DFT methods, the chemical potential of electrons and ionic species that participate in elementary steps must be evaluated. Several DFT modeling approaches have been developed to include the influence of solvent and/or electrochemical potential on surface reactions and to take into account the chemical potential of ionic species. 

---