Ab initio computational packages

The ab initio methods use no experimental parameters in their computations but are much more onerous than the semiempirical ones, both for time consuming and for hard disk capacity requirements. The onerousness increases on increasing the molecular dimensions and the level of sophistication, i.e., the extension of the basis set adopted for calculations. Among the most used ab initio computation packages, we remember here the GAUSSIAN 03 [122], GAMESS [123] and SPARTAN [124]. 

Even if basis set descriptions are easily available on all the user manuals accompanying ab initio computation package, we will give here some short news on the most used bases ... 

The Hartree-Fock method for periodic systems nowadays represents a routine approach coded in several ab initio computer packages. We may analyze the total energy, its dependence on molecular conformation, the density of states, the atomic charges, etc. Also, calculations of first-order responses to the electric field (poljmiers are of interest for optoelectronics) have been successful in the past. However, nonlinear problems (like the second harmonic generation see Chapter 12) stiU represent a challenge. 

A number of software packages to perform real-time computational experiments are available. Programs such as HYPERCHEM or UNICHEM, as used in the lectures introduced above (see Tables 1 and 2), are very useful to illustrate CC lectures in real time. Whereas HYPERCHEM is used to illukrate MM, MD (Figure 2), and semiempirical methods, UNICHEM is employed to provide practical examples of problem solving using ab initio computation. 

The properties of these systems are obtained from published experimental data and/or the electronic structure calculations. For crystalline structures, the electronic structure calculations are performed with plane-wave DFT calculations using the software VASP (Vienna Ab initio Simulation Package) with appropriate pseudopotential and exchange-correlation functionals (US-LLDA or PAW-PBE). For molecular systems, the electronic structure calculations are performed using the Gaussian09 computational chemistry software package. Table 7.1 shows the fitting database and predicted values from the COMB potential for Cu. 

Jmol can be also used to animate the results of simulations that are in a multiframe XYZ format and to animate the computed normal modes from ab initio quantum chemistry packages. 

Equation (2) was also used to calculate quantum chemical approach. On the basis of previous results [19], calculated electrostatic potentials were computed from ab initio wave functions obtained in the framework of the HF/SCF method using a split-valence basis set (3-21G) and a split-valence basis set plus polarisation functions on atoms other than hydrogen (6-31G ). The GAUSSIAN 90 software package [20] was used. Since ab initio calculations of the molecular wave function for the whole... 

The electronic structure method used to provide the energies and gradients of the states is crucial in photochemistry and photophysics. Ab initio electronic structure methods have been used for many years. Treating closed shell systems in their ground state is a problem that, in many cases, can now be solved routinely by chemists using standardized methods and computer packages. In order to obtain quantitative results, electron correlation (also referred to as dynamical correlation) should be included in the model and there are many methods available for doing this based on either variational or perturbation principles [41],... 

Ab-initio CAChe features all of the above plus ab-initio and density functional methods. This program requires a workstation (Windows NT minimum or SGI and IBM unix-based machines) and can be used to build and visualize results from ab-initio programs (e.g., Gaussian, see description under Gaussian, Inc.). Also, CAChe directly interfaces to Dgauss , a computational chemistry package that uses density functional theory to predict molecular structures, properties, and energetics. 

Recently, quantum chemical computational techniques, such as density functional theory (DFT), have been used to study the electrode interface. Other methods ab initio methods based on Hartree-Fock (HF) theory,65 such as Mollcr-PIcsset perturbation theory,66,67 have also been used. However, DFT is much more computationally efficient than HF methods and sufficiently accurate for many applications. Use of highly accurate configuration interaction (Cl) and coupled cluster (CC) methods is prohibited by their immense computational requirements.68 Advances in computing capabilities and the availability of commercial software packages have resulted in widespread application of DFT to catalysis. 

The study of the enantioselective hydrosilylation reaction was performed with a series of combined quantum mechanics/molecular mechanics (QM/MM) calculations [26, 30] within the computational scheme of ab initio (AIMD) (Car-Parrinello) [62] molecular dynamics. The AIMD approach has been described in a number of excellent reviews [63-66], AIMD as well as hybrid QM/MM-AIMD calculations [26, 47] were performed with the ab initio molecular dynamics program CPMD [67] based on a pseudopotential framework, a plane wave basis set, and periodic boundary conditions. We have recently developed an interface to the CPMD package in which the coupling with a molecular mechanics force field has been implemented [26, 68],... 

Hypercube, Inc. at http //www.hyper.com offers molecular modeling packages under the HyperChem name. HyperChem s newest version, Hyper-Chem Release 7.5, is a full 32-bit application, developed for the Windows 95, 98, NT, ME, 2000, and XP operating systems. Density Functional Theory (DFT) has been added as a basic computational engine to complement Molecular Mechanics, Semiempirical Quantum Mechanics and ab initio Quantum Mechanics. The DFT engine includes four combination or hybrid functions, such as the popular B3-LYP or Becke-97 methods. The Bio+ force field in HyperChem represents a version of the Chemistry at HARvard using Molecular Mechanics (CHARMM) force field. Release 7.5 of HyperChem updates... 

In this paper we present the first application of the ZORA (Zeroth Order Regular Approximation of the Dirac Fock equation) formalism in Ab Initio electronic structure calculations. The ZORA method, which has been tested previously in the context of Density Functional Theory, has been implemented in the GAMESS-UK package. As was shown earlier we can split off a scalar part from the two component ZORA Hamiltonian. In the present work only the one component part is considered. We introduce a separate internal basis to represent the extra matrix elements, needed for the ZORA corrections. This leads to different options for the computation of the Coulomb matrix in this internal basis. The performance of this Hamiltonian and the effect of the different Coulomb matrix alternatives is tested in calculations on the radon en xenon atoms and the AuH molecule. In the atomic cases we compare with numerical Dirac Fock and numerical ZORA methods and with non relativistic and full Dirac basis set calculations. It is shown that ZORA recovers the bulk of the relativistic effect and that ZORA and Dirac Fock perform equally well in medium size basis set calculations. For AuH we have calculated the equilibrium bond length with the non relativistic Hartree Fock and ZORA methods and compare with the Dirac Fock result and the experimental value. Again the ZORA and Dirac Fock errors are of the same order of magnitude. 

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